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.

1. To act as a substitute for; SYN. deputize, stand in, step in.
2. To be a substitute; SYN. sub, stand in, fill in.
3. To put in the place of another; SYN. replace.
In mathematics, to put values in the place of variables in an algebraic expression or formula. An algebraic expansion or simplification can be checked by substituting simple values for each variable. For example, to check that
X3 + y3 = (x + y)(x2 + y2 - xy)
The value x = 1 and y = 2 might be substituted in both sides of the expression, giving
Left-hand side: 13 + 23 = 1 + 8 = 9
Right-hand side: (1 + 2)(12 + 22 - 2) = 3 x 3 = 9.
The two sides are the same, so the expansion of x3 + y3 is correct.