a person or thing acting or serving in place of another.

(formerly) a person who, for payment, served in an army or navy in the place of a conscript.

Grammar . a word that functions as a replacement for any member of a class of words or constructions, as do in He doesn't know but I do.

to put (a person or thing) in the place of another.

to take the place of; replace.

Chemistry . to replace (one or more elements or groups in a compound) by other elements or groups.

to act as a substitute.

of or pertaining to a substitute or substitutes.

composed of substitutes.

noun

commutation , exchange , interchange , shift , switch , trade , transposition