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A catkin or ament is a slim, cylindrical flower cluster, with inconspicuous or no petals, usually wind-pollinated (anemophilous) but sometimes insect pollinated (as in Salix). They contain many, usually unisexual flowers, arranged closely along a central stem which is often drooping. They are found in many plant families, including Betulaceae, Fagaceae, Moraceae, and Salicaceae. For some time, they were believed to be a key synapomorphy among the proposed Hamamelididae, but it is now believed that this flower arrangement has arisen independently by convergent evolution on a number of occasions.

In many of these plants only the male flowers form catkins, and the female flowers are single (hazel, oak), a cone (alder) or other types (mulberry). In other plants (such as poplar) both male and female flowers are borne in catkins.

Catkin-bearing plants include many other trees or shrubs such as birch, willow, hickory, sweet chestnut and sweetfern (Comptonia), and also some herbaceous plants such as nettle.

The word catkin is derived from the Dutch katje, meaning "kitten", on account of the resemblance to a kitten's tail.^[1] Ament is from the Latin amentum, meaning "thong" or "strap".^[2]

Male catkins of Ostrya carpinifolia   A male catkin on a willow (Salix sp.)   A male flowering catkin on a willow (Salix sp.)   Male Hazel catkins (Corylus avellana)   Male flowering catkin on a Goat Willow (Salix caprea)