To substitute means to use one thing or person in place of another. When your regular teacher is sick, a substitute teacher takes over the class. When you run out of butter while baking cookies, you might substitute oil or applesauce instead.

The word works as both a verb and a noun. As a verb, you substitute honey for sugar in a recipe. As a noun, the honey becomes the substitute for sugar. In sports, a coach might send in a substitute player when the starter needs a rest.

A good substitute serves the same purpose as the original, even if it works a bit differently. Lemon juice can substitute for vinegar in salad dressing because both add tanginess. But you wouldn't substitute salt for sugar in cookies since they taste completely different and would ruin the recipe.

Substitution is the act or process of substituting, like when a coach makes a substitution by swapping one basketball player for another during a timeout.

The word suggests both replacement and equivalence: you're removing something and putting something workable in its place. When mathematicians substitute values into an equation or when actors substitute for each other during rehearsals, they're making a trade that lets the work continue.