In mathematics:
* Substitution in algebra: Replacing a variable (or an expression) with another value or expression. This is done to simplify an equation, solve for a variable, or evaluate an expression. For example, in the equation `y = 2x + 1`, substituting `x = 3` gives us `y = 2(3) + 1 = 7`.
* Substitution in calculus: A method of finding the integral of a function by replacing the variable with a new variable, which simplifies the integral.
* Substitution in logic: Replacing a propositional variable with a specific proposition. For example, in the statement "If P then Q", substituting "P" with "It is raining" and "Q" with "The ground is wet" results in "If it is raining then the ground is wet".
In other contexts:
* Substitution in chemistry: Replacing one atom or group of atoms in a molecule with another. For example, in the process of halogenation, hydrogen atoms in a molecule are substituted by halogen atoms.
* Substitution in linguistics: Replacing a word or phrase with another word or phrase that has a similar meaning. This is often used for stylistic reasons, or to avoid repetition.
* Substitution in sports: Replacing one player with another during a game.
The specific definition of substitution will depend on the context in which the term is being used. If you can provide more details about the context, I can give you a more precise definition.