In medicine a spasm is a sudden, involuntary contraction of a muscle, a group of muscles, or a hollow organ, or a similarly sudden contraction of an orifice. It most commonly refers to a muscle cramp which is often accompanied by a sudden burst of pain, but is usually harmless and ceases after a few minutes. There is a variety of other causes of involuntary muscle contractions, which may be more serious, depending on the cause. The word "spasm" may also refer to a temporary burst of energy, activity, emotion, stress, or anxiety unrelated to, or as a consequence of, involuntary muscle activity.

A variety of types of involuntary muscle activity may be referred to as a "spasm". Examples include muscle contractions due to abnormal nerve stimulation, or abnormal activity of the muscle itself. A series of spasms or permanent spasms are called a spasmism. A spasm may lead to muscle strains or tears of tendons and ligaments, if the force of the spasm exceeds the tensile strength of the underlying connective tissues, such as with a particularly forceful spasm, or in the case of weakened connective tissues.