Substitution means replacing one thing with another. When your teacher is sick and a substitute teacher takes over the class, that's substitution in action. The original teacher is temporarily replaced by someone who can do the same job.

In sports, substitution happens constantly. A basketball coach might pull a tired player out and send in a fresh substitute from the bench. In soccer, teams have a limited number of substitutions they can make during a game, so coaches have to think carefully about when to make each substitution.

The word appears in math too. In algebra, you might use substitution to replace a variable with its value: if x equals 5, you can substitute 5 wherever you see x in an equation.

Substitution can be temporary or permanent. You might substitute honey for sugar in a recipe because you ran out, or substitute one activity for another when your first choice isn't available. The key is that the substitute serves the same purpose or fills the same role as whatever it replaced, even if it's not exactly identical.