Source : Webster's Revised Unabridged Dictionary (1913)

Day lily \Day" lil`y\ (l[i^]l`[y^]). (Bot.)
      (a) 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.
      (b) A genus of plants ({Funkia}) differing from the last
          in having ovate veiny leaves, and large white or blue
          flowers.

Source : WordNet®

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}]
     2: any of numerous perennials having tuberous roots and long
        narrow bladelike leaves and usually yellow lilylike
        flowers that bloom for only a day [syn: {daylily}]