The Collaborative International Dictionary of English v.0.48:

day lily \day" lil`y\, daylily \day"lil`y\(l[i^]l`[y^]). (Bot.)
      (a) any plant of a genus of plants (Hemerocallis)
          closely resembling true lilies, but having tuberous
          rootstocks instead of bulbs. The common species have
          long narrow leaves and either yellow or tawny-orange
          flowers, which often bloom for only one day.
      (b) A genus of plants (Funkia) differing from the last
          in having ovate veiny leaves, and large white or blue
          flowers.
          [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

day lily
    n 1: any of numerous perennials having mounds of sumptuous broad
         ribbed leaves and clusters of white, blue, or lilac
         flowers; used as ground cover [syn: plantain lily, day
         lily]
    2: any of numerous perennials having tuberous roots and long
       narrow bladelike leaves and usually yellow lily-like flowers
       that bloom for only a day [syn: day lily, daylily]