sac·cade 
(să-käd, sə-)
(să-käd, sə-)n.
A rapid intermittent eye movement, as that which occurs when the eyes fix on one point after another in the visual field.
[French, jerk, jolt, from Middle French, from Old French dialectal saquer, to jerk, pull, variant of Old French sachier, to pull forcefully, turn over, shake up, from sac, sack (perhaps in reference to the shaking of bags of wheat to settle their contents); see SAC1.]
sac·cadic adj.