Source : Webster's Revised Unabridged Dictionary (1913)

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}.]

Source : WordNet®

Listera
     n : genus of terrestrial orchids having usually a single pair of
         broad shining leaves near the middle of the stem; found
         in temperate Asia and North America and Europe [syn: {genus
         Listera}]