The Collaborative International Dictionary of English v.0.48:

Twayblade \Tway"blade`\, n. (Bot.)
   Any one of several orchidaceous plants which have only two
   leaves, as the species of Listera and of Liparis.
   [Written also twyblade.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

twayblade
    n 1: orchid having a pair of ovate leaves and a long slender
         raceme of green flowers sometimes tinged red-brown; Europe
         to central Asia [syn: twayblade, Listera ovata]
    2: an orchid of the genus Liparis having a pair of leaves