A catkin is a slim flower cluster (a "spike") which is usually wind-pollinated. Sometimes it is insect-pollinated. They have many flowers arranged closely along a drooping central stem.^[1] Usually, they are unisexual.

They are found in many plant families, including the Betulaceae, Fagaceae, Moraceae, and Salicaceae. The catkin flower arrangement has arisen at least twice independently by convergent evolution, in the Fagales and in Salicaceae.