Chief is a rank. Some people in starfleet belive, that "Chief" is short for one of the ranks of the non-commissioned officers or the head of a department, but that are only the guys that are called "Chief".

Every other person in starfleet knows, that "Chief" is the secret (not that secret) nick for "That guy that was once a Lieutenant and then got demoted but he wasn't told so and we gave him this title so that he will never notice and now he has to do all the work on this junkyard".

This is an article only a mother could love - too bad it's an orphan!