A sink was a basin, usually located in the bathroom or a kitchen area, in which water and soap could be used to wash one's hands, dishes, or even clothes.
Two sinks were part of the equipment of an NX-class sickbay. (ENT: "Marauders", "Regeneration")
The galley aboard Enterprise NX-01 also featured a sink. (ENT: "The Catwalk")
A sink was present in the brig aboard Starfleet ships. (Star Trek V: The Final Frontier)