1An opening through a floor or series of floors, especially one into which a staircase, chimney stack, etc., may be installed, or one which admits light.
2A central open space forming the vertical core around which the stairs of a winding or spiral staircase may be set. Also more generally: the whole shaft in which such a staircase is set; a stairwell.
3A vertical shaft in a building for a lift or for lifting equipment.
4A vertical hole or shaft in the ground; specifically the shaft or borehole of a well.
Mid 17th century. From well + hole.