The Collaborative International Dictionary of English v.0.48:

Hemerocallis \Hem`e*ro*cal"lis\, n. [NL., fr. Gr. ?; ? day +
   ka`llos beauty.] (Bot.)
   A genus of plants, some species of which are cultivated for
   their beautiful flowers; day lily.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Hemerocallis
    n 1: east Asian rhizomatous clump-forming perennial herbs having
         flowers on long leafless stalks; cosmopolitan in
         cultivation: day lilies; sometimes placed in subfamily
         Hemerocallidaceae [syn: Hemerocallis, genus
         Hemerocallis]