What is the X in X-mas?
1886, medical Latin, from Greek sklera (menix) "the hard (membrane)," fem. of skleros "hard" (see sclerosis).
sclera scle·ra (sklēr'ə)n. pl. scle·ras or scle·rae (sklēr'ē) The tough fibrous tunic forming the outer envelope of the eye and covering all of the eyeball except the cornea; the white of the eye. Also called sclerotic.