1.The action of replacing one thing or person with another: The principal wanted to have Mr. Jones as the substitution for Mrs. Hathaway, but Mr. Jones wasn't available after all.
2. That which is replaced: When at the restaurant, Jill wanted to have vegetables instead of french fries, but the waiter said substitutions were not allowed.
3. An individual or thing that acts as an alternative or replacement: Dr. Good proposed a cast for Jim's broken arm as a substitution for surgery and a stay at the hospital which appealed to Jim very much!