We need more context, but here are some definitions for substitution:
Substitution: the action of replacing someone or something with another person or thing.
Substitution effect: The substitution effect is the decrease in sales for a product that can be attributed to consumers switching to cheaper alternatives when its price rises.
But you asked this in chemistry so I think you may be looking for:
Substitution reaction: A substitution reaction is a chemical reaction during which one functional group in a chemical compound is replaced by another functional group. Substitution reactions are of prime importance in organic chemistry.