Crewman

Crewman (also known as Ables'man or Able Crewman) is an enlisted naval rate, a type of military rank used by the service organizations of different cultures. This title is the lowest enlisted rank, subordinate to a Petty Officer. In comparison to infantry ranking systems, this rank is approximately equivalent to the grade of Private. In Klingonese, this rank is referred to as bekk.

In Starfleet, this rank is awarded upon completion of a training course, where recruits are given basic instruction in manning a starship or filling another role in the fleet.

From Memory Alpha -- the free Star Trek reference