The Collaborative International Dictionary of English v.0.48:

Umbel \Um"bel\, n. [L. umbella a little shadow, umbrella, dim.
   of umbra shade. See Umbrella.] (Bot.)
   A kind of flower cluster in which the flower stalks radiate
   from a common point, as in the carrot and milkweed. It is
   simple or compound; in the latter case, each peduncle bears
   another little umbel, called umbellet, or umbellule.
   [1913 Webster]

WordNet (r) 3.0 (2006):

umbel
    n 1: flat-topped or rounded inflorescence characteristic of the
         family Umbelliferae in which the individual flower stalks
         arise from about the same point; youngest flowers are at
         the center