What is the X in X-mas?
prefix meaning "beyond" (ultraviolet) or "extremely" (ultramodern), from Latin ultra- from ultra (adv. and prep.) "beyond, on the further side," from PIE *al- "beyond." In common use from early 19c., it appears to have arisen from French political designations. As its own word, a noun meaning "extremist" of various stripes, it is first recorded 1817, from French ultra, shortening of ultra-royaliste "extreme royalist."
Beyond; on the other side of: ultraviolet.
Beyond the range, scope, or limit of: ultrasonic.