What is the X in X-mas?
late 14c., from Old French cenith (Modern French zénith), from Medieval Latin cenit, senit, bungled scribal transliterations of Arabic samt "road, path," abbreviation of samt ar-ras, literally "the way over the head." Letter -m- misread as -ni-.
The Medieval Latin word could as well be influenced by the rough agreement of the Arabic term with classical Latin semita "sidetrack, side path" (notion of "thing going off to the side"), from se- "apart" + *mi-ta-, suffixed zero-grade form of PIE root *mei- "to change" (see mutable).
The point on the celestial sphere that is directly above the observer (90 degrees above the celestial horizon). Compare nadir.
point on the celestial sphere directly above an observer on the Earth. The point 180 opposite the zenith, directly underfoot, is the nadir. Astronomical zenith is defined by gravity; i.e., by sighting up a plumb line. If the line were not deflected by such local irregularities in the Earth's mass as mountains, it would point to the geographic zenith. Because the Earth rotates and is not a perfect sphere, the geocentric zenith is slightly different from the geographic zenith except at the Equator and the poles. Geocentric zenith is the intersection with the celestial sphere of a straight line drawn through the observer's position from the geometric centre of the Earth