[lu.men] n, pl lumens also lu.mi.na [NL lumin-, lumen, fr. L, light, air shaft, opening] (1873) 1: the cavity of a tubular organ "the ~ of a blood vessel"
2: the bore of a tube (as of a hollow needle or catheter)
3: a unit of luminous flux equal to the light emitted in a unit solid angle by a uniform point source of one candle intensity -- lu.mi.nal also lu.men.al adj