1. The act of furnishing an equivalent person or thing in the place of another; SYN. replacing.
2. One that replaces another especially in a job or function

ETYM Latin substitutus, p. p. of substituere to put under, put in the place of; sub under + statuere to put, place: cf. French substitut. Related to Statute.
1. A person or thing that takes or can take the place of another; SYN. replacement.
2. An athlete who plays only when another member of the team drops out; SYN. reserve.