Pinnation (also called pennation) is the arrangement of feather-like or multi-divided features arising from both sides of a common axis. Pinnation occurs in biological morphology, in crystals, such as some forms of ice or metal crystals, and in patterns of erosion or stream beds.

The term derives from the Latin word pinna meaning "feather", "wing", or "fin". A similar concept is "pectination," which is a comb-like arrangement of parts (arising from one side of an axis only). Pinnation is commonly referred to in contrast to "palmation," in which the parts or structures radiate out from a common point. The terms "pinnation" and "pennation" are cognate, and although they are sometimes used distinctly, there is no consistent difference in the meaning or usage of the two words.