What is the X in X-mas?
"partition between the nostrils," 1690s, Modern Latin, from Latin saeptum "a fence, enclosure, partition," from neuter past participle of saepire "to hedge in," from saepes "hedge, fence." Related: Septal.
septum sep·tum (sěp'təm)
n. pl. sep·ta (-tə)
A thin partition or membrane dividing two cavities or soft masses of tissue in an organism.
The septum pellucidum.