The Collaborative International Dictionary of English v.0.48:

Oncidium \On*cid"i*um\, n. [NL.] (Bot.)
   A genus of tropical orchidaceous plants, the flower of one
   species of which (Oncidium Papilio) resembles a butterfly.
   [1913 Webster]

WordNet (r) 3.0 (2006):

oncidium
    n 1: any orchid of the genus Oncidium: characterized by slender
         branching sprays of small yellow and brown flowers; often
         grown as houseplants [syn: oncidium, dancing lady
         orchid, butterfly plant, butterfly orchid]