The Collaborative International Dictionary of English v.0.48:

Quince \Quince\ (kw[i^]ns), n. [Prob. a pl. from OE. quyne,
   coin, OF. coin, cooin, F. coing, from L. Cydonius a quince
   tree, as adj., Cydonian, Gr. ? Cydonian, ? ? a quince, fr. ?
   Cydonia, a city in Crete, ? the Cydonians. Cf. Quiddany.]
   1. The fruit of a shrub (Cydonia vulgaris) belonging to the
      same tribe as the apple. It somewhat resembles an apple,
      but differs in having many seeds in each carpel. It has
      hard flesh of high flavor, but very acid, and is largely
      used for marmalade, jelly, and preserves.
      [1913 Webster]

   2. (Bot.) a quince tree or shrub.
      [1913 Webster]

   Japan quince (Bot.), an Eastern Asiatic shrub (Cydonia
      Japonica, formerly Pyrus Japonica) and its very
      fragrant but inedible fruit. The shrub has very showy
      flowers, usually red, but sometimes pink or white, and is
      much grown for ornament.

   Quince curculio (Zool.), a small gray and yellow curculio
      (Conotrachelus crat[ae]gi) whose larva lives in quinces.


   Quince tree (Bot.), the small tree (Cydonia vulgaris)
      which produces the quince.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Cydonin \Cy*do"nin\ (s?-d?"n?n), n. (Chem.)
   A peculiar mucilaginous substance extracted from the seeds of
   the quince (Cydonia vulgaris), and regarded as a variety of
   amylose.
   [1913 Webster]