A tic is a suden, repetitive, stereotyped, nonrhythmic movement (motor tic) or sound (phonic tic) that involves discrete groups of muscles. Tics can be invisible to the observer (e.g.; abdominal tensing or toe crunching). Movements of other movement disorders (e.g.; chorea, dystonia, myoclonus) must be distinguished from tics. Other conditions (e.g.; autism, stereotypic movement disorder) also include movements which may be confused with tics. Tics must also be distinguished from compulsions of OCD and seizure activity.