a beginning from which an enterprise is launched; "he uses other people's ideas as a springboard for his own"; "reality provides the jumping-off point for his illusions"; "the point of departure of international comparison cannot be an institution but must be the function it carries out"
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