I. noun 1. a space created by a window line projecting outwards from a wall. 2. a section of wall between two buttresses or columns, especially in the nave of a church. 3. [with modifier] — a compartment with a specified function in a vehicle, aircraft, or ship • a bomb bay.
4. an area specially allocated or marked off • a loading bay.
5. (also bay platform) —
(Brit.) a short terminal platform at a railway station also having through lines.
– origin late Middle English: from Old French baie, from baer ‘to gape’, from medieval Latin batare, of unknown origin.