The Collaborative International Dictionary of English v.0.48:

Pappus \Pap"pus\, n. [L., an old man or grandfather; hence, a
   substance resembling gray hairs, Gr. ?.] (Bot.)
   The hairy or feathery appendage of the achenes of thistles,
   dandelions, and most other plants of the order
   Composit[ae]; also, the scales, awns, or bristles which
   represent the calyx in other plants of the same order.
   [1913 Webster]

WordNet (r) 3.0 (2006):

pappus
    n 1: calyx composed of scales or bristles or featherlike hairs
         in plants of the Compositae such as thistles and dandelions