The Collaborative International Dictionary of English v.0.48:

Corolla \Co*rol"la\ (k?-r?l"l?), n. [L. corolla a little crown
   or garland, dim. of corona. See Crown.] (Bot.)
   The inner envelope of a flower; the part which surrounds the
   organs of fructification, consisting of one or more leaves,
   called petals. It is usually distinguished from the calyx by
   the fineness of its texture and the gayness of its colors.
   See the Note under Blossom.
   [1913 Webster]

WordNet (r) 3.0 (2006):

corolla
    n 1: (botany) the whorl of petals of a flower that collectively
         form an inner floral envelope or layer of the perianth; "we
         cultivate the flower for its corolla"