The Collaborative International Dictionary of English v.0.48:

Michaelmas \Mich"ael*mas\, n. [Michael + mass religious service;
   OE. Mighelmesse.]
   The feast of the archangel Michael, a church festival,
   celebrated on the 29th of September. Hence, colloquially,
   autumn.
   [1913 Webster]

   Michaelmas daisy. (Bot.) See under Daisy.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Daisy \Dai"sy\ (d[=a]"z[y^]), n.; pl. Daisies (d[=a]"z[i^]z).
   [OE. dayesye, AS. d[ae]ges-e['a]ge day's eye, daisy. See
   Day, and Eye.] (Bot.)
   (a) A genus of low herbs (Bellis), belonging to the family
       Composit[ae]. The common English and classical daisy is
       Bellis perennis, which has a yellow disk and white or
       pinkish rays.
   (b) The whiteweed (Chrysanthemum Leucanthemum), the plant
       commonly called daisy in North America; -- called also
       oxeye daisy. See Whiteweed.
       [1913 Webster]

   Note: The word daisy is also used for composite plants of
         other genera, as Erigeron, or fleabane.
         [1913 Webster]

   Michaelmas daisy (Bot.), any plant of the genus Aster, of
      which there are many species.

   Oxeye daisy (Bot.), the whiteweed. See Daisy
   (b) .
       [1913 Webster] daisybush

WordNet (r) 3.0 (2006):

Michaelmas daisy
    n 1: North American perennial herb having small autumn-blooming
         purple or pink or white flowers; widely naturalized in
         Europe [syn: Michaelmas daisy, New York aster, Aster
         novi-belgii]