In mathematics:

In economics:

In medicine:

In Analytic psychology:

Substitution cipher, in cryptography

Substitution (chemistry), in organic chemistry

The substitution method of solving simultaneous equations

Import substitution in trade

Substitution rule, in calculus

String substitution, in theoretical computer science

Chord substitution, in music

Substitute good, in consumer theory

Substitution (law), a legal right to change a judge that may be biased.

Substitution method, in the testing of optical fiber

Substitution (theatre), an acting methodology

Substitutes, in sport

Substitution therapy for opiates

Variable substitution, in first-order logic

In science and technology:

Substitution mutation is synonymous with point mutation in genetics

Virtual Reality as a substitute for Reality

It is a defence mechanism where a person replaces one feeling or emotion for another.

Substitutionary Covenantal Representation in Biblical Theology