Euphemism is the use of a word (or phrase) that replaces another one that one thinks to be too offensive or vulgar.

It also may be a replacement of a name or a word, that could reveal a secret or holy and sacred names to the uninitiated. It may also be used to obscure the identity of the subject of a conversation from potential eavesdroppers. A well known example for many is the replacement of Lord Voldemort's name by "You-Know-Who" or "He-Who-Must-Not-Be-Named" in Harry Potter. Some euphemisms are intended to be humorous.