From the Latin “pinna,” meaning “feather, wing, fin,” this adjective describes something that resembles a feather, meaning it has parts arranged on either side of an axis. Think of a spine with wisps projecting out from the center — that’s pinnate. While a bird’s feathers are the root of this word, you’ll most often see it referring to plants. Many ferns have pinnate structures, with fluffy leaves extending out from a spine.