Formal:
* Archaic: This word emphasizes that the word or phrase is very old and no longer used in modern language.
* Obsolete: This implies that the word or phrase is completely out of use and no longer understood by most people.
* Antiquated: Similar to "archaic," but might also apply to ideas or practices that are no longer relevant.
Informal:
* Outdated: This word is more general and can be used for words, phrases, or even technology that is no longer current.
* Passé: This word suggests that something is no longer fashionable or in style.
* Dead: This is a very informal and slangy way to say that a word or phrase is no longer used.
Other options:
* Disused: This word emphasizes that the word or phrase was once used but is no longer in use.
* Deprecated: This word is often used in the context of computer programming to describe features that are no longer recommended for use.
The best word to use will depend on the specific context and the level of formality you want to achieve.