Is when each plaintext element or group of elements is uniquely replaced by a corresponding ciphertext element or group of elements.

Substitution is when each plaintext element or group of elements is uniquely replaced by a corresponding ciphertext element or group of elements.

The word "Substitution" would signify when things would be replaced either for a little while, or even forever. So, in this aspect, this would also be known to be called a "Substitution".