1. A form of movement marked by contractions and relaxations of a muscle, occurring in rapid succession.
2. A continuous rhythmic reflex tremor initiated by the spinal cord below an area of spinal cord injury, set in motion by reflex testing.
3. Spasmodic alternation of muscular contractions between antagonistic muscle groups caused by a hyperactive stretch reflex from an upper motor neuron lesion.

Usually, sustained pressure, or stretch of one of the muscles, inhibits the reflex.

An extended clarification about clonus

The term clonus is a succession of intermittent muscular relaxations and contractions, usually resulting from a sustained stretching stimulus.

An example is the clonus stimulated in the calf muscle by maintaining sustained upward pressure on the sole of the foot. The condition is often a sign of disease in the brain or spinal cord.

A series of abnormal reflex movements of the foot, induced by sudden dorsiflexion, causing alternate contractions and relaxations of the triceps surae muscle.

A rhythmic jerking movement of the patella ("kneecap" or the small bone that is in the front of the knee) produced by grasping it between the thumb and the forefinger and pushing it suddenly and forcibly toward the foot.

This is an abnormal reflex with alternate contractions and relaxations of the quadriceps muscle (great extensor muscle of the knee, forming a large fleshy mass which covers the front and sides of the femur or thigh bone).

Spasmodic contraction of the hand muscles, induced by forcibly extending the hand at the wrist.