What is the X in X-mas?
element active in English word formation from mid-18c. and meaning "star or celestial body; outer space," from Greek astro-, stem and comb. form of astron "star," related to aster "star" (see star (n.)). In ancient Greek, aster typically was "a star" and astron mostly in plural, "the stars." In singular it mostly meant "Sirius" (the brightest star).
A prefix that means "star" (as in astrophysics), "celestial body" (as in astronomy), or "outer space" (as in astronaut).