The Collaborative International Dictionary of English v.0.48:

Polyanthus \Pol`y*an"thus\ (p[o^]l`[i^]*[a^]n"th[u^]s), n.; pl.
   Polyanthuses. [NL., fr. Gr. poly`anqos rich in flowers;
   poly`s many + 'a`nqos flower.] [Written also polyanthos.]
   (Bot.)
   (a) The oxlip. So called because the peduncle bears a
       many-flowered umbel. See Oxlip.
   (b) A bulbous flowering plant of the genus Narcissus
       (Narcissus Tazetta, or Narcissus polyanthus of some
       authors). See Illust. of Narcissus.
       [1913 Webster]

WordNet (r) 3.0 (2006):

polyanthus
    n 1: florists' primroses; considered a complex hybrid derived
         from oxlip, cowslip, and common primrose [syn:
         polyanthus, Primula polyantha]