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
the occurrence of incoming water (between a low tide and the following high tide); "a tide in the affairs of men which, taken at the flood, leads on to fortune" -Shakespeare