The Collaborative International Dictionary of English v.0.48:

Stipule \Stip"ule\, n. [L. stipula a stalk, stem, straw: cf. F.
   stipule. Cf. Stubble.] (Bot.)
   An appendage at the base of petioles or leaves, usually
   somewhat resembling a small leaf in texture and appearance.
   [1913 Webster]

WordNet (r) 3.0 (2006):

stipule
    n 1: a small leafy outgrowth at the base of a leaf or its stalk;
         usually occurring in pairs and soon shed