Guilt refers simply a negative affective state in which one experiences regret at having done something one believes one "should not" have done. Guilt and its causes, merits, and demerits is a common theme in psychology and psychiatry. It is often associated with depression.
Other /More definition:
Guilt is defined as the feeling that results most intensely from living an inauthentic life. It is an unpleasant moral emotion associated with a specific instance in which one has acted badly or wrongly Guilt are thoughts and feelings that assign blame, usually self-blame, fault, or culpability for a loss or death; an emotion associated with doing something wrong or the anticipation of doing something wrong.