A tic is a sudden and repetitive motor movement or vocalization that is not rhythmic and involves discrete muscle groups. It is typically brief, and may resemble a normal behavioral characteristic or gesture.
Tics can be invisible to the observer, such as abdominal tensing or toe crunching. Common motor and phonic tics are, respectively, eye blinking and throat clearing.
Tics must be distinguished from movements of disorders such as chorea, dystonia and myoclonus; the compulsions of obsessive–compulsive disorder (OCD) and seizure activity; and movements exhibited in stereotypic movement disorder or among autistic people (also known as stimming).
Tics are classified as either motor or phonic, and simple or complex.
Motor tics are movement-based tics affecting discrete muscle groups.
Phonic tics are involuntary sounds produced by moving air through the nose, mouth, or throat. They may be alternately referred to as verbal tics or vocal tics, but most diagnosticians prefer the term phonic tics to reflect the notion that the vocal cords are not involved in all tics that produce sound.
Simple motor tics are typically sudden, brief, meaningless movements that usually involve only one group of muscles, such as eye blinking, head jerking, or shoulder shrugging. Motor tics can be of an endless variety and may include such movements as hand clapping, neck stretching, mouth movements, head, arm or leg jerks, and facial grimacing.
A simple phonic tic can be almost any sound or noise, with common vocal tics being throat clearing, sniffing, or grunting.
Complex motor tics are typically more purposeful-appearing and of a longer nature. They may involve a cluster of movements and appear coordinated. Examples of complex motor tics are pulling at clothes, touching people, touching objects, echopraxia (repeating or imitating another person's actions) and copropraxia (involuntarily performing obscene or forbidden gestures).