What is a letter or symbol that replaces?

A letter or symbol that replaces another is often referred to as a "substitute" or "replacement." In linguistics, this can occur in phonetics where one sound replaces another, or in cryptography where symbols replace letters in a code. For example, in algebra, variables like "x" or "y" can replace numbers in equations. In informal writing, "@" can replace "a" in certain contexts, such as in email addresses.