Definition of alternates:
(adj) :
Happening by turns; one following the other in succession of time or place; first one and then the other (repeatedly).
(adj) :
(mathematics) Designating the members in a series, which regularly intervene between the members of another series, as the odd or even numbers of the numerals; every other; every second.
(adj) :
(US) Other; alternative.
(n) :
(US) A substitute; an alternative; one designated to take the place of another, if necessary, in performing some duty.
(v) :
(transitive) To perform by turns, or in succession; to cause to succeed by turns; to interchange regularly.
(v) :
(intransitive) To happen, succeed, or act by turns; to follow reciprocally in place or time; followed by with.