type of compound leaf. with leaflets along each side of a common axis (contrast with bipinnate and palmate).

Merriam-Webster Online Dictionary
pinnate (adjective)
resembling a feather especially in having similar parts arranged on opposite sides of an axis like the barbs on the rachis of a feather - a pinnate leaf see leaf illustration
pinnate (Wikipedia)
A simple pinnate (unipinnate) frond of the fern, Blechnum appendiculatum.

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.

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