What is the X in X-mas?
1570s, from Middle French convexe, from Latin convexus "vaulted, arched," past participle of convehere "to bring together," from com- "together," or "thoroughly" (see com-) + vehere "to bring" (see vehicle). Possibly from the idea of vaults carried together to meet at the point of a roof. Related: Convexity. Convex lens is from 1822.
convex con·vex (kŏn'věks', kən-věks')
Having a surface or boundary that curves or bulges outward, as the exterior of a sphere.