What is the X in X-mas?
early 15c., "creed, summary, religious belief," from Late Latin symbolum "creed, token, mark," from Greek symbolon "token, watchword" (applied c.250 by Cyprian of Carthage to the Apostles' Creed, on the notion of the "mark" that distinguishes Christians from pagans), literally "that which is thrown or cast together," from syn- "together" (see syn-) + bole "a throwing, a casting, the stroke of a missile, bolt, beam," from bol-, nominative stem of ballein "to throw" (see ballistics).
The sense evolution in Greek is from "throwing things together" to "contrasting" to "comparing" to "token used in comparisons to determine if something is genuine." Hence, "outward sign" of something. The meaning "something which stands for something else" first recorded 1590 (in "Faerie Queene").
symbol sym·bol (sĭm'bəl)
Something that represents something else by association, resemblance, or convention, especially a material object used to represent something invisible.
A printed or written sign used to represent an operation, an element, a quantity, or a relation, as in mathematics or chemistry.
A conventional sign.
An object or name that stands for something else, especially a material thing that stands for something that is not material. The bald eagle is a symbol of the United States of America. The cross is a symbol of Christianity. The Star of David is a symbol of Judaism.
Something that represents or suggests something else. Symbols often take the form of words, visual images, or gestures that are used to convey ideas and beliefs. All human cultures use symbols to express the underlying structure of their social systems, to represent ideal cultural characteristics, such as beauty, and to ensure that the culture is passed on to new generations. Symbolic relationships are learned rather than biologically or naturally determined, and each culture has its own symbols.