What is the X in X-mas?
late 14c., "a cutting made in surgery," from Old French incision (13c.) and directly from Latin incisionem (nominative incisio) "a cutting into," noun of action from past participle stem of incidere "to cut in," from in- "into, in, on, upon" (see in- (2)) + -cidere, comb. form of caedere "to cut" (see -cide). Meaning "act of cutting into" is from early 15c.
incision in·ci·sion (ĭn-sĭzh'ən)
A cut into a body tissue or organ, especially one made during surgery.
The scar resulting from such a cut.