A cramp is a sudden, involuntary, painful skeletal muscle contraction or overshortening associated with electrical activity; while generally temporary and non-damaging, they can cause significant pain and a paralysis-like immobility of the affected muscle. A cramp usually goes away on its own over a period of several seconds, or minutes. Cramps are common and tend to occur at rest, usually at night (nocturnal leg cramps). They are also often associated with pregnancy, physical exercise or overexertion, age (common in older adults), in such cases, cramps are called idiopathic, because there is no underlying pathology. In addition to those benign conditions cramps are also associated with many pathologic conditions.

Cramp definition is narrower than the definition of muscle spasm: spasms include any involuntary abnormal muscle contractions, while cramps are sustained and painful. True cramps can be distinguished from other cramp-like conditions. Cramps are different from muscle contracture, which is also painful and involuntary, but which is electrically silent. The main distinguishing features of cramps from dystonia are suddenness with acute onset of pain, involvement of only one muscle and spontaneous resolution of cramps or their resolution after stretching the affected muscle. Restless leg syndrome is not considered the same as muscle cramps and should not be confused with rest cramps.