Other forms: restrooms
Restroom is the American word for a public toilet. The sign beside the door in an airport, a diner, a shopping mall.
It is a polite substitute, and America keeps several in rotation: restroom, bathroom, washroom, the ladies' room. British English says toilet or loo and finds restroom faintly odd, since nobody goes there to rest. If you are travelling in the United States, asking for the toilet will be understood but marks you instantly as a visitor. The word means the room itself, not the fixture inside it.
From rest + room, from an earlier sense of break room.
Etymology from Wiktionary, CC BY-SA