The Collaborative International Dictionary of English v.0.48:

Corymb \Cor"ymb\ (k?r"?mb or -?m; 220), n. [L. corymbus cluster
   of flowers, Gr. ko`rymbos.] (Bot.)
   (a) A flat-topped or convex cluster of flowers, each on its
       own footstalk, and arising from different points of a
       common axis, the outermost blossoms expanding first, as
       in the hawthorn.
   (b) Any flattish flower cluster, whatever be the order of
       blooming, or a similar shaped cluster of fruit.
       [1913 Webster]

WordNet (r) 3.0 (2006):

corymb
    n 1: flat-topped or convex inflorescence in which the individual
         flower stalks grow upward from various points on the main
         stem to approximately the same height; outer flowers open
         first