the world > time > change > exchange > substitution > supplanting or replacement > [noun] > one who or that which replaces (4)
A thing that replaces or supersedes another.
A person who or thing which replaces another, esp. as a substitute.
Something which takes the place of another; a replacement, a substitute.
Linguistics. A morph or morpheme that consists of or is realized by the replacement of one form by another.