Someone who fights for money.
Mercenaries are important in drow-society, albeit not very respectable. They sell their skills to the highest bidder and are used to beef up a House expecting an attack or preparing for one. Rather than having a too large, too expensive standing force it is better to hire mercenaries.
Most who become mercenaries are houseless males (normally survivors from the fall of one noble House or the other) but commoner-females are not uncommon.
It is also an odd social structure within a band of mercenaries since they are normally led by males, rather than females further adding to their lack of respectability in drow-society.
Generally mercenaries are semi-nomadic, drifting from one job to another withing any given city. In some cases they also move between cities to look for employment, usually alongside merchant caravans, making mercenaries a source for news and gossip between cities.
Mercenary Companies of note: