Is it ensure, insure, or assure?
1550s, from Latin rotationem (nominative rotatio) "a turning about in a circle," noun of action from past participle stem of rotare "turn round, revolve, whirl about, roll," from PIE *roto- (see rotary).
rotation ro·ta·tion (rō-tā'shən)
The act or process of turning around a center or an axis.
Regular and uniform variation in a sequence or series, as in the recurrence of symptoms of a disease.