Chief petty officer

Chief Petty Officer (CPO) is an enlisted naval rate, a type of military rank used by the service organizations of different cultures. As a traditional naval grade, CPO is senior to a Petty Officer, and is the highest enlisted grade in organizations like Starfleet and the US Navy. In comparison to infantry ranking systems, this rank is approximately equivalent to the more senior Sergeant grades. Personnel of this grade can be addressed simply as chief, and sometimes by their job title, such as "chief specialist."

While chiefs are subordinate to all commissioned officers, they have far more experience and practical knowledge than Ensigns and Lieutenants, and are expected to act as mentors to them and to the junior enlisted assigned. As such, chiefs are often among the actual leaders in a Starfleet organization. Chiefs serve with distinction in most Starfleet branches and specialties, but are particularly visible in engineering and other technical trades. Wise is the ensign who heeds the advice of his chief, woe betide the ensign who disregards it.

From Memory Alpha -- the free Star Trek reference