What is the X in X-mas?
c.1600, "state of standing still; firmness," from Middle French consistence (Modern French consistance) "a standing fast," from Medieval Latin consistentia, from Latin consistentem (nominative consistens), present participle of consistere (see consist). Meaning "coherence, solidity" is recorded from 1620s.
1590s, "firmness of matter," from Medieval Latin consistentia or directly from Latin consistentem, from consistere (see consist). Meaning "state of being in agreement or harmony" (with something) is from 1650s; meaning "self-consistent" is from 1716.