Definition of "rachis"

rachis

/ˈɹeɪkɪs/

noun
plural rachises or rachides
1. (obsolete, zoology, anatomy) The spinal column, or the vertebrae of the spine.
2. (zoology) An anatomical shaft or axis in a marine invertebrate.
  Quotations
  In some squid species, the pen rachis flares out into wings on either side, and the tip of the rachis grows into a thick cone.
  2017, Danna Staaf, Squid Empire, ForeEdge, page 115
3. (ornithology) The central shaft of a feather.
4. (botany) The main shaft of either a compound leaf, head of grain, or fern frond.