Muscle twitch is a term used to describe two different types of involuntary muscle contractions. The first type is known as a SPASM, which is a sudden, involuntary contraction of a muscle or group of muscles. The second type is known as a TIC, which is a repetitive, involuntary contraction of a muscle or group of muscles. Both spasms and tics can cause pain and discomfort, and can be caused by a variety of medical conditions.