n. 1 a space created by a window-line projecting outwards from a wall. 2 a recess; a section of wall between buttresses or columns, esp. in the nave of a church etc. 3 a compartment (bomb bay). 4 an area specially allocated or marked off (sick bay; loading bay). 5 Brit. the terminus of a branch line at a railway station also having through lines, usu. at the side of an outer platform. bay window a window built into a bay.
[ ME f. OF baie f. ba(y)er gape f. med.L batare ]