The Collaborative International Dictionary of English v.0.48:

Lucern \Lu"cern\, n. [F. luzerne.] (Bot.)
   A leguminous plant (Medicago sativa), having bluish purple
   cloverlike flowers, cultivated for fodder; -- called also
   alfalfa. [Written also lucerne.]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Alfalfa \Al*fal"fa\, n. [Sp.] (Bot.)
   The lucern (Medicago sativa), a leguminous plant having
   bluish purple cloverlike flowers, and cultivated for fodder;
   -- so called in California, Texas, etc.
   [1913 Webster]

WordNet (r) 3.0 (2006):

alfalfa
    n 1: important European leguminous forage plant with trifoliate
         leaves and blue-violet flowers grown widely as a pasture
         and hay crop [syn: alfalfa, lucerne, Medicago sativa]
    2: leguminous plant grown for hay or forage