Final answer:
The process described is an example of a single substitution cipher, where each letter is replaced by another letter a fixed number of positions away in the alphabet.
Step-by-step explanation:
Shifting each letter in the alphabet a fixed number of spaces to the right or left is an example of a single substitution cipher. In this type of cipher, each letter in the plaintext is replaced by a letter some fixed number of positions down or up the alphabet. This method is a type of substitution cipher where each letter corresponds to a single letter throughout the message. It is not to be confused with a bit shifting, confusion, or multi-substitution cipher.