alternation (n.)

"act of alternating; state of being alternate," mid-15c., alternacioun, from Old French alternacion "alternation," from Latin alternationem (nominative alternatio) "an interchanging," noun of action from past-participle stem of alternare "to do first one thing then the other; exchange parts," from alternus "one after the other, alternate, in turns, reciprocal," from alter "the other" (see alter).