The word is derived from Latin: dissocire, dissocit- : dis-, dis- + socire, to unite (from socius, companion; (social ally, companion).
The meaning of Dissociation is; 1) The disintegergration of the self in different segments, 2) The separate occurence of bodily phenomena, that normally appear connected, 3) (chemical) The fission of molecules into smaller constituents

The French psychiatrist Pierre Janet (1859-1947) coined the word dissaggregation about one hundred years ago to identify changes in consciousness which disturbed the normal, well-integrated functions of identity, memory and thought in several of his patients. This term was later translated from the French as dissociation. Janet's studies of patients with amnesias, fugues, and 'successive existences' (now known as other personalities), convinced him that their symptoms were the effect of split-off parts of the personality which were capable of independent thoughts, actions and identities. Further, he concluded that the dissociation which caused the symptoms was the result of past traumatic experiences, and that the symptoms could be alleviated by bringing the split-off memories and feelings into consciousness.

Confusion and wrong words;
There was a lot of confusion about the dissociative disorders and when it's pathological. In the '80 one still thought that light forms of dissociation (f.i. highway trance) and severe forms of pathological dissociation processes existed on a continuum. But not anymore cause they are different processes. Cause by the former so called normal dissociation, the ego (or ego-state) stays intact, which is not the case by dissociation. So, daydreaming, trance like states and highway trance, are not dissociation! Also ego-states (f.i. acting differently at work than at home or country house) is not dissociation, but is a part of normal human psyche. So there are numerous examples where the word 'dissociation' is misused or used in the wrong context.
Like when sometimes the wrong word of 'disassociation' (opposite to 'association') is used where one means 'dissociation'. The meaning of 'disassociation' is the state of being unconnected in memory or imagination; F.i.; "I could not think of him in disassociation from his wife".

One can speak of dissociative symptoms when for instance: someone is told that he or she has a serious disease or experience a life-threatening accident, he or she can temporary dissociate (f.i.: feeling detached from other people and reality = (depersonalization/derealization), as a reaction to the acute stress or trauma. In those cases one can speak of dissociative phenomena and it is a natural way of coping with acute stress.

• So dissociation literal means; 'experiencing detachment from one self, experiencing a reality outside one self' or 'a falling apart of the self in segments or personality parts' (also called alters), who have amnesia for each other.
• Alters are not ego states. Having egostates is normal. Between the alters (from the word; alter ~ change, changing) there is amnesia.
• Somatoform Dissociation. Dissociation can also relate to physical functions, so that paralysis (conversion) other motoric disfunctions or unsensitive sensory perceptions can arise, of which no physical cause can be found.

There is a spectrum of dissociative disorders (which are all trauma-based): see the spectrum van de dissociative disorders.
The most serious and pathological form of dissociation is the Dissociative Identity Disorder (DID, former called the Multiple Personality Disorder (MPD).

Speaking of dissociative disorders nowadays one means pathological, like in symptoms of a disease. So at first, dissociation is a natural reaction to overwhelming traumatic events or circumstances. It becomes pathology (a disease) when a patient dissociates for a long time period and in that case one speaks of a dissociative disorder. So a dissocciative disorder is a disease, (other than a dissociative symptom).
In the meantime several researchers found that evidence, that dissociation is connected with all kinds of traumatic experiences in childhood; like abuse, (affect) neglect and sexual abuse. And one also found that adults who react on childhood burdens with dissociation, have a greater chance to develop a Post Traumatic Disorder (PTSD).

The Dissociative Disorders according to the DSM IV:
1. dissociative amnesia
2. dissociative fugue
3. derealization - depersonalization (DP)
4. dissociative disorder not otherwise specified (DDNOS)
5. dissociatieve identity disorder (DID)

Dissociative Disorders according to the ICD 10:

F44 Dissociative [conversion] disorders
F44.0 Dissociative amnesia
F44.1 Dissociative fugue
F44.2 Dissociative stupor
F44.3 Trance and possession disorders
F44.4 Dissociative motor disorders
F44.5 Dissociative convulsions
F44.6 Dissociative anaesthesia and sensory loss
F44.7 Mixed dissociative [conversion] disorders
F44.8 Other dissociative [conversion] disorders
  .80 Ganser's syndrome
  .81 Multiple personality disorder
  .82 Transient dissociative [conversion] disorders occurring in childhood and adolescence
  .88 Other specified dissociative [conversion] disorders
F44.9 Dissociative [conversion] disorder, unspecified

Note:
In many cases the DP is a secundairy symptom of different primairy (personality)-disorders, like the Dissociative Identity Disorder and the Borderline Syndrome.

By some researchers and therapists the DID is seen at the same continuum as the borderline syndrome. In spite of obvious differences between the two and there are also a lot of scientific studies which demonstrate clear physiological and psychological differences and outcomes in psychological tests, between a DID and a BPD. So there are a lot of misdiagnosis.
For example; ego states are not dissociatied alters and alters are no moodswings and dissociation is a different process than repression. In genuine cases of DID there is fear (or phobia) and embarrasment about having other personalities. "In contrast, those individuals who show up on TV talk shows, touting their “diagnosis,” raise the most suspicion of having ulterior motives, such as a craving for attention and money, to be seen by others as “special” and different". Raymond Lloyd Richmond, Ph.D.

1. The predominant disturbance is one or more episodes of inability to recall important personal information, usually of a traumatic or stressful nature, that is too extensive to be explained by ordinary forgetfulness.
2. The disturbance does not occur exclusively during the course of Dissociative Identity Disorder, Dissociative Fugue, Posttraumatic Stress Disorder, Acute Stress Disorder, or Somatization Disorder and is not due to the direct physiological effects of a substance (for example, a drug of abuse, a medication) or a neurological or other general medical condition (for example (Amnestic Disorder Due to Head Trauma).
3. The symptoms cause clinically significant distress or impairment in social, occupational, or other important areas of functioning.

1. The predominant disturbance is sudden, unexpected travel away from home or one's customary place of work, with inability to recall one's past.
2. Confusion about personal identity or assumption of a new identity (partial or complete).
3. The disturbance does not occur exclusively during the course of Dissociative Identity Disorder and is not due to the direct physiological effects of a substance (for example, a drug of abuse, a medication) or a general medical condition (for example, temporal lobe epilepsy).
4. The symptoms cause clinically significant distress or impairment in social, occupational, or other important areas of functioning.

1. Clinical presentations similar to Dissociative Identity Disorder that fail to meet full criteria for this Disorder. Examples include presentations in which
   • there are not two or more distinct personality states, or
   • amnesia for important personal information does not occur.
2. De-realization unaccompanied by depersonalization in adults.
3. States of dissociation that occur in individuals who have been subjected to periods of prolonged and intense coercive persuasion (e.g., brainwashing, thought re-form, or indoctrination while captive).
4. Dissociative trance disorder: single or episodic disturbances in the state of consciousness, identity, or memory that are indigenous to particular locations and cultures. Dissociative trance involves narrowing of awareness of immediate surroundings or stereotyped behaviors or movements that are experienced as being beyond one's control.
   • Possession trance involves replacement of the customary sense of personal identity by a new identity, attributed to the influence of a spirit, power, deity, or other person, and associated with stereotyped "involuntary" movements or amnesia. Examples include amok (Indonesia), bebainan (Indonesia), latab (Malaysia), pibloktoq (Arctic), ataque de nervios (Latin America), and possession (India). The Dissociative or trance disorder is not a normal part of a broadly accepted collective cultural or religious practice.
5. Loss of consciousness, stupor, or coma not attributable to a general medical condition.
6. Ganser syndrome: the giving of approximate answers to questions (e.g., "2 plus 2 equals 5") when not associated with Dissociative Amnesia or Dissociative Fugue. (DSM-IV)

The most severe form of the dissociative disorders.
A disorder which is often faked. Only about 6% of people diagnosed with some kind of dissociative disorder does have a real or flamboyant D.I.D. (Kluft 1985c)

Dissociating is is something like creating a distance between yourself and the outside world. Cause of fragmentation in different personality parts or alters, someone with a DID experiences above all dissociations in her or himself.
In relation with the outside world, some or all of the above mentioned dissociative symptoms can occur within a patient with DID (former called Multiple Personality Disorder MPD).

The term MPD is no longer mentioned in the DSM IV (now DID), but still in the ICD 10 from the World Health Organization.

The DSM IV criteria are:

1. The presence of two or more distinct identities or personality states (each with its own relatively enduring pattern of perceiving, relating to, and thinking about the environment and self).
2. At least two of these identities or personality states recurrently take control of the person's behavior.
3. Inability to recall important personal information that is too extensive to be explained by ordinary forgetfulness.
4. The disturbance is not due to the direct physiological effects of a substance (e.g., blackouts or chaotic behavior during Alcohol Intoxication) or a general medical condition (e.g., complex partial seizures). Note: In children, the symptoms are not attributable to imaginary playmates or other fantasy play.
   
   see for more: what is did/mpd?

1. The person has been exposed to a traumatic event in which both of the following have been present:
   • the person experienced, witnessed, or was confronted with an event or events that involved actual or threatened death or serious injury, or a threat to the physical integrity of self or others
   • the person's response involved intense fear, helplessness, or horror. Note: In children, this may be expressed instead by disorganized or agitated behavior.
2. The traumatic event is persistently reexperienced in one (or more) of the following ways:
   • recurrent and intrusive distressing recollections of the event, including images, thoughts, or perceptions. Note: In young children, repetitive play may occur in which themes or aspects of the trauma are expressed.
   • recurrent distressing dreams of the event. Note: In children, there may be frightening dreams without recognizable content.
   • acting or feeling as if the traumatic event were recurring (includes a sense of reliving the experience, illusions, hallucinations, and dissociative flashback episodes, including those that occur upon awakening or when intoxicated). Note: In young children, trauma-specific reenactment may occur.
   • intense psychological distress at exposure to internal or external cues that symbolize or resemble an aspect of the traumatic event.
   • physiological reactivity on exposure to internal or external cues that symbolize or resemble an aspect of the traumatic event.
3. Persistent avoidance of stimuli associated with the trauma and numbing of general responsiveness (not present before the trauma), as indicated by three (or more) of the following:
   • efforts to avoid thoughts, feelings, or conversations associated with the trauma
   • efforts to avoid activities, places, or people that arouse recollections of the trauma
   • inability to recall an important aspect of the trauma
   • markedly diminished interest or participation in significant activities
   • feeling of detachment or estrangement from others
   • restricted range of affect (e.g., unable to have loving feelings)
   • sense of a foreshortened future (e.g., does not expect to have a career, marriage, children, or a normal life span)
4. Persistent symptoms of increased arousal (not present before the trauma), as indicated by two (or more) of the following:
   • difficulty falling or staying asleep
   • irritability or outbursts of anger
   • difficulty concentrating
   • hypervigilance
   • exaggerated startle response
5. Duration of the disturbance (symptoms in Criteria B, C, and D) is more than one month.
6. The disturbance causes clinically significant distress or impairment in social, occupational, or other important areas of functioning.

Not listed in the dissociative disorders, but has much to do with dissociative phenomenon.

Catatonia: A condition of diminished responsiveness usually characterized by trancelike states and by a waxy rigidity of the muscles (flexibilitas cerea) so that the patient tends to remain in any position in which he is placed; it occurs in organic and psychological disorder and under hypnosis. Also called anochlesia.