ra·chis 
(rākĭs)
(rākĭs)n. pl. ra·chis·es or rach·i·des (răkĭ-dēz′, rākĭ-)
1.
a. The main stem of an elongated inflorescence, as in a grass.
b. The main axis of a pinnately compound leaf or of a fern frond.
2. The main shaft of a bird's feather, especially the part to which the barbs are attached.
3. The spinal column.
[New Latin, from Greek rhakhis, spine, ridge.]
rachi·al adj.