Substitution can refer to different concepts depending on the context. Here are some possible meanings:

• Mathematics: Substitution is the replacement of one mathematical entity by another of equal value. In algebra, the substitution method is one of the algebraic methods to solve simultaneous equations. It involves substituting the value of any one of the variables from one equation into the other equation to find the solutions of the variables.

• Grammar: In English grammar, substitution is the replacement of a word or phrase with a filler word to avoid repetition. For example, instead of saying "John went to the store, and John bought some milk," we can say "John went to the store, and he bought some milk." The pronoun "he" substitutes for the name "John."

• Consumer theory: In economics, a substitute or substitutable good is a product or service that consumers see as essentially the same or similar enough to another product. For example, Coke and Pepsi are substitutes for each other because they can be used for the same purpose, which is to quench thirst.

• Other meanings: Substitution can also refer to other concepts, such as an answer or solution for all problems or difficulties, a stronghold, or a complete extent or range.

In summary, substitution can mean different things depending on the context, but it generally involves replacing one thing with another that is equivalent or similar.