The Collaborative International Dictionary of English v.0.48:

LEM \LEM\, LEM \L.E.M.\n. [acronym from Lunar Excursion Module.]
   (Astronautics)
   A lunar excursion module, a spacecraft designed to
   transport people from a command module in orbit around the
   moon to the surface of the moon and back again.

   Syn: lunar excursion module, lunar module.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

LEM
    n 1: a spacecraft that carries astronauts from the command
         module to the surface of the moon and back [syn: lunar
         excursion module, lunar module, LEM]