Crewman is a generic term for a crew member serving in the operation of an aircraft, naval vessel, or train. The term may also refer to individuals serving in a military capacity on weapon system platforms, such as those operating a tank.
In some science fiction (most notably Star Trek), crewman is the lowest military rank on board a spacecraft, analogous to seaman in many real world navies.
The term "crewman" may also be used interchangeably with the non-gender specific form as "crewperson" or "crewmember".
References
Crewman Wikipedia(Text) CC BY-SA