a cavity or passage in a tubular organ; "the lumen of the intestine"
a unit of luminous flux equal to the amount of light given out through a solid angle of 1 steradian by a point source of 1 candela intensity radiating uniformly in all directions
having little or no weight or apparent gravitational pull; light; "floating freely in a weightless condition"; "a baby bat...fluffy and weightless as a moth"; "jackets made of a weightless polyester fabric"