In general, substitution is the replacement of one thing with another. There are many specific examples:
- In organic chemistry, a reaction where one functional group takes the place of another. A major type of substitution is nucleophilic substitution.
- The substitution cipher in cryptography.
- The substitution rule in calculus.
- Import substitution in trade.
- The substitution method in the testing of optical fiber.
- The substitution property of equality in mathematics.