Substitution

noun
1.
a person or thing acting or serving in place of another.
2.
(formerly) a person who, for payment, served in an army or navy in the place of a conscript.
3.
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.
verb (used with object), substituted, substituting.
4.
to put (a person or thing) in the place of another.
5.
to take the place of; replace.
6.
Chemistry. to replace (one or more elements or groups in a compound) by other elements or groups.
verb (used without object), substituted, substituting.
7.
to act as a substitute.
adjective
8.
of or relating to a substitute or substitutes.
9.
composed of substitutes.
substitution
/ˌsʌbstɪˈtjuːʃən/
noun
1.
the act of substituting or state of being substituted
2.
something or someone substituted
3.
(maths) the replacement of a term of an equation by another that is known to have the same value in order to simplify the equation
4.
(maths, logic)

the uniform replacement of one expression by another
substitution instance, an expression so derived from another

verb
1.
(often foll by for) to serve or cause to serve in place of another person or thing
2.
(chem) to replace (an atom or group in a molecule) with (another atom or group)
3.
(logic, maths) to replace (one expression) by (another) in the context of a third, as replacing x + y for x in 3x = k gives 3x + 3y = k
noun
4.

a person or thing that serves in place of another, such as a player in a game who takes the place of an injured colleague
(as modifier): a substitute goalkeeper Often shortened to sub

5.
(grammar) another name for pro-form
6.
(Canadian) another name for supply teacher
7.
(nautical) another word for repeater (sense 5)
8.
(formerly) a person paid to replace another due for military service

substitution sub·sti·tu·tion (sŭb’stĭ-tōō’shən, -tyōō’-)
n.

The replacement of an atom or group of atoms in a compound by another atom or group of atoms.

An unconscious defense mechanism by which the unacceptable or unattainable is replaced by something more acceptable or attainable.