n. - The spine.
n. - The continued stem or midrib of a pinnately compound leaf, as in a rose leaf or a fern.
n. - The principal axis in a raceme, spike, panicle, or corymb.
n. - The shaft of a feather. The rhachis of the after-shaft, or plumule, is called the hyporhachis.
n. - The central cord in the stem of a crinoid.
n. - The median part of the radula of a mollusk.
n. - A central cord of the ovary of nematodes.