Source : Webster's Revised Unabridged Dictionary (1913)

Catkin \Cat"kin\, 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}.

Source : WordNet®

catkin
     n : a cylindrical spikelike inflorescence [syn: {ament}]