The Collaborative International Dictionary of English v.0.48:

Mariposa lily \Ma`ri*po"sa lil`y\ [Sp. mariposa a butterfly + E.
   lily. So called from the gay appearance of the blossoms.]
   (Bot.)
   One of a genus (Calochortus) of tuliplike bulbous herbs
   with large, and often gaycolored, blossoms. Called also
   butterfly lily. Most of them are natives of California.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mariposa lily
    n 1: any of several plants of the genus Calochortus having
         tulip-shaped flowers with 3 sepals and 3 petals;
         southwestern United States and Mexico [syn: mariposa,
         mariposa tulip, mariposa lily]