Dictionary.net

Habitual, repeated, rapid contraction of certain muscles, resulting in stereotyped individualized actions that can be voluntarily suppressed for only brief periods. They often involve the face, vocal cords, neck, and less often the extremities. Examples include repetitive throat clearing, vocalizations, sniffing, pursing the lips, and excessive blinking. Tics tend to be aggravated by emotional stress. When frequent they may interfere with speech and INTERPERSONAL RELATIONS. Conditions which feature frequent and prominent tics as a primary manifestation of disease are referred to as TIC DISORDERS. (From Adams et al., Principles of Neurology, 6th ed, pp109-10)