Successive change or variation resulting from the alternate occurrence of phenomena; an instance of this. Obsolete.

An instance of alternation or succession; a change from one physical state to another, esp. as one of a constant series.

or something (colloquial), used to express an indistinct or unknown alternative.

A situation or arrangement in which two or more people take turns in occupying the same space or position, sustaining a part in some activity, etc.