A dead key is a key combination that triggers a dead key state. It can be any key you like, with any combination of modifiers you like.

An early convention with the Mac was to make dead keys ordinary keys with the option key, such as option-u for diaresis, option-e for acute accent, option-n for tilde, or option-` for grave accent. However, this is just a convention, and you are free to use whatever you like.