Adjustment Disorder refers to a type of mental disorder which resulted from maladaptive, or unhealthy, responses to stressful or psychologically distressing life events.

This low level of adaptation then leads to the development of emotional or behavioral symptoms. An adjustment disorder represents significant difficulty in adjusting to the new reality.
The stressful events that precipitate an Adjustment Disorder are varied, which may include the loss of a job; the end of a romantic relationship; a life transition such as a career change or retirement; a serious accident or sickness; or some stressors like relocating to a new area. DSM-IV-TR specifies six (6) subtypes of Adjustment Disorder, each with its own predominant symptoms:

(1) With depressed mood: The chief manifestations are feelings of sadness and depression, with a sense of accompanying hopelessness. The patient may be tearful and have uncontrollable bouts of crying.
(2) With anxiety: The patient is troubled by feelings of apprehension, nervousness, and worry. He or she may also feel jittery and unable to control his or her thoughts of doom. Children with this subtype may express fears of separation from parents or other significant people, and refuse to go to sleep alone or attend school.
(3) With mixed anxiety and depressed mood: The patient has a combination of symptoms from the previous two (2) subtypes.
(4) With disturbance of conduct: This subtype involves such noticeable behavioral changes as shoplifting, truancy, reckless driving, aggressive outbursts, or sexual promiscuity. The patient disregards the rights of others or previously followed rules of conduct with little concern, guilt or remorse.
(5) With mixed disturbance of emotions and conduct: The patient exhibits sudden changes in behavior combined with feelings of depression or anxiety. He or she may feel or express guilt about the behavior, but then repeat it shortly thereafter.
(6) Unspecified: This subtype covers patients who are adjusting poorly to stress but who do not fit into the other categories. These patients may complain of physical illness and pull away from social contact.

Adjustment Disorders which can can occur at any stage of life. may lead to suicide or suicidal thinking. They may also complicate the treatment of other diseases when, for instance, a sufferer loses interest in taking medication as prescribed or adhering to diets or exercise regimens.

Other definition:
Adjustment disorder refers to a DSM-IV classification designating the development of clinically significant symptoms in response to stress in which the symptoms are not severe enough to warrant classification as another mental disorder.

Other definition:

adjustment disorder refers to stress-related disorder that involves emotional and behavioral symptoms (depressive symptoms, anxiety symptoms, and/or antisocial behaviors ) that arise within three (3) months of the onset of a stressor