Petty officer

Petty Officer is an enlisted naval rate, a type of military rank used by the service organizations of different cultures. This title is senior to a Crewman, and subordinate to a Chief Petty Officer. In comparison to infantry ranking systems, this rank is approximately equivalent to the grade of Corporal. However, the Petty Officer 2nd Class rank is equivalent to that of Sergeant and the 1st Class rank is equivalent to that of Staff Sergeant.

This is the lowest rank considered to have the responsibility of a "noncommissioned Officer." Personnel of this grade are usually referred to by their job title. In Starfleet, a petty Officer could be referred to as yeoman, or as specialist.

From Memory Alpha -- the free Star Trek reference