Webster's English Dictionary

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lumen

lu.men also lu.men.al \'lu:-m*n\ \-m*-n*\ \-m*n-*l\ n or lu.mi.na or lumens 
   [NL lumin-, lumen, fr. L, light, airshaft, opening] pl  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 - 
   lu.min.al aj