spasm, involuntary rigid muscle contraction, often persistent and often accompanied by pain. It usually has some underlying physical cause such as disease, strain, or injury to the muscle or nearby tissues, impairment of circulation, or a disturbance of body chemistry. The spasm may be confined to one group of muscles or it may be severe and fairly generalized, as in convulsions. Painless localized spasms are called tics. These purposeless movements, usually of some part of the face, may begin as purposeful movement in response to some stimulus but eventually are carried out automatically, apparently without reason. They may disappear spontaneously after a time, or may require the elimination of some physical or psychic cause.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.