rachis

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The main stem of an elongated inflorescence, as in a grass.
• noun The main axis of a pinnately compound leaf or of a fern frond.
• noun The main shaft of a bird's feather, especially the part to which the barbs are attached.
• noun The spinal column.

from The Century Dictionary.

• noun In botany:
• noun The axis of an inflorescence when somewhat elongated; the continuation of the peduncle along which the flowers are ranged, as in a spike or a raceme.
• noun In a pinnately compound leaf or frond, the prolongation of the petiole along which the leaflets or pinnæ are disposed, corresponding to the midrib of a pinnately veined simple leaf. See cut under compound.
• noun In zoöl. and anatomy:
• noun The vertebral column.
• noun The stem, shaft, or scape of a feather, as distinguished from the web, vane, or vexillum; especially, that part of the stem which bears the vexillum. as distinguished from the calamus or quill. See quill, 4.
• noun The median part of the radula of a mollusk, usually bearing teeth which differ from those on each side of it.
• noun The axial skeleton of various polyp-colonies, as of Gorgonia; some axial part, or formation like a midrib, as in crinoids.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Anat.) The spine; the vertebral column.
• noun (Bot. & Zoöl.) Same as Rhachis.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun botany The main shaft of either a compound leaf, head of grain, or fern frond.
• noun zoology, anatomy The spine or the vertebrae of the spine.
• noun ornithology The central shaft of a feather.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun axis of a compound leaf or compound inflorescence
• noun the series of vertebrae forming the axis of the skeleton and protecting the spinal cord