late 14c., "terror," from Old French tremor "fear, terror" (13c.), from Latin tremorem (nominative tremor) "a trembling, terror," from tremere (see tremble). Sense of "an involuntary shaking" first recorded 1610s and probably represents a re-introduction from Latin.
tremor trem·or (trěm'ər)
An involuntary trembling movement.
Minute ocular movement occurring during fixation on an object.