means A set of stairs, usually with its supporting framework and railings, built to connect one level of a building to another.

from A plain compound of "stair" + "case." "Stair" comes from Old English "stǣger," tied to a verb meaning to climb or mount (a cousin of "stile" and German "steigen," to ascend). "Case" here is the older sense of an enclosing frame or structure — the same "case" that boxes a window or a clock — from Latin "capsa," a box, via Old French. So a staircase is literally the climbing-thing housed in its frame.