(1):
(n.) An opening, space, or cavity, esp. a tubular cavity; a vacuole.
(2):
(n.) A unit of illumination, being the amount of illumination of a unit area of spherical surface, due to a light of unit intensity placed at the center of the sphere.
(3):
(n.) A unit of light flux, being the flux through one square meter of surface the illumination of which is uniform and of unit brightness.