An involuntary,constant,rhythmic movement of the eyeball is called:

A) strabismus.
B) trichiasis.
C) retinitis pigmentosa.
D) nystagmus.

Nystagmus

Involuntary rhythmic movement of the eyes, which can impair vision and can be a sign of neurological problems.

Involuntary

Actions or processes not under the conscious control of an individual, such as breathing or the heartbeat.

Rhythmic Movement

Repeated motions that follow a certain pattern in time, often seen in dance, exercise, and various biological processes.