Sudden, repetitive, involuntary muscular movement or vocal pattern.

Tic disorders feature involuntary repetitive (but non-rhythmic) patterns, and may be either motor tics (muscle movements) or vocal tics. Although tics are involuntary, the individual with a tic disorder can often repress the tic for a period of time. The occurrences of tics appear to be more likely when the individual is under stress or concentrating on a task, such as reading or writing. Most tics seem to nearly disappear during sleep.

The Diagnostic and Statistical Manual of Mental Disorders, 4th edition, lists the following examples:

Both motor and vocal tics may be categorized as simple or complex, although the distinction between the two is not precise. Generally, a simple motor tic involves only one part of the body, while a complex tic is more involved and takes the form of some recognizable action. They also include imitating...