The Collaborative International Dictionary of English v.0.48:

Carpel \Car"pel\ (k[aum]r"p[e^]l), Carpellum \Car*pel"lum\
   (-p[e^]l"l[u^]m), n. [NL. carpellum, fr. Gr. karpo`s fruit.]
   (Bot.)
   A simple pistil or single-celled ovary or seed vessel, or one
   of the parts of a compound pistil, ovary, or seed vessel. See
   Illust of Carpaphore.
   [1913 Webster]

WordNet (r) 3.0 (2006):

carpel
    n 1: a simple pistil or one element of a compound pistil