What is the X in X-mas?
late 15c., "interior, inward, internal," from Middle French intrinsèque "inner" (13c.), from Medieval Latin intrinsecus "interior, internal," from Latin intrinsecus (adv.) "inwardly, on the inside," from intra "within" (see intra-) + secus "alongside," originally "following" (related to sequi "to follow;" see sequel). Meaning "belonging to the nature of a thing" is from 1640s. Related: Intrinsicly.
intrinsic in·trin·sic (ĭn-trĭn'zĭk, -sĭk)
Of or relating to the essential nature of a thing.
Situated within or belonging solely to the organ or body part on which it acts. Used of certain nerves and muscles.