What is the X in X-mas?
see encyclopedia. The Latin spelling survives as a variant because many of the most prominent ones (e.g. Britannica) have Latin names.
1530s, "course of instruction," from Modern Latin encyclopaedia (c.1500), thought to be a false reading by Latin authors of Greek enkyklios paideia taken as "general education," but literally "training in a circle," i.e. the "circle" of arts and sciences, the essentials of a liberal education; from enkyklios "circular," also "general" (from en "in" + kyklos "circle") + paideia "education, child-rearing," from pais (genitive paidos) "child" (see pedo-).
Modern sense of "reference work arranged alphabetically" is from 1640s, often applied specifically to the French "Encyclopédie ou Dictionnaire raisonné des Sciences, des Arts, et des Métiers" (1751-65).