The Collaborative International Dictionary of English v.0.48:

Cuckoobud \Cuck"oo*bud"\ (k??k"??-b?d`), n. (Bot.)
   A species of Ranunculus (Ranunculus bulbosus); -- called
   also butterflower, buttercup, kingcup, goldcup.
   --Shak.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Buttercup \But"ter*cup`\, n. (Bot.)
   A plant of the genus Ranunculus, or crowfoot, particularly
   Ranunculus bulbosus, with bright yellow flowers; -- called
   also butterflower, golden cup, and kingcup. It is the
   cuckoobud of Shakespeare.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Ranunculus bulbosus
    n 1: perennial Old World buttercup with golden to sulphur yellow
         flowers in late spring to early summer; naturalized in
         North America [syn: common buttercup, Ranunculus
         bulbosus]