The Collaborative International Dictionary of English v.0.48:

Catkin \Cat"kin\ (k[a^]t"k[i^]n), n. [Cat + -kin.] (Bot.)
   An ament; a species of inflorescence, consisting of a slender
   axis with many unisexual apetalous flowers along its sides,
   as in the willow and poplar, and (as to the staminate
   flowers) in the chestnut, oak, hickory, etc. -- so called
   from its resemblance to a cat's tail. See Illust. of Ament.
   [1913 Webster]

WordNet (r) 3.0 (2006):

catkin
    n 1: a cylindrical spikelike inflorescence [syn: catkin,
         ament]