Biology A main axis or shaft, such as the main stem of an inflorescence, the stalk of a pinnately compound leaf, or the spinal column.(noun)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "rachis" in a sentence
"Pinnately compound leaves have the leaflets arranged along the main or mid-vein (called a rachis in this case). odd pinnate: with a terminal leaflet, e.g. Fraxinus (ash). even pinnate: lacking a terminal leaflet, e.g. Swietenia (mahogany)."
"Zoom out, and we see that feathers have a central shaft called the rachis with two vanes on either side."
"The spikelets are unilaterally biseriate on the rachis which is not jointed at the base."