What is the X in X-mas?
1620s, from Late Latin hyphen, from Greek hyphen "mark joining two syllables or words," probably indicating how they were to be sung, noun use of an adverb meaning "together, in one," literally "under one," from hypo "under" (see sub-) + hen, neuter of heis "one."
A punctuation mark (-) used in some compound words, such as self-motivation, seventy-five, and mother-in-law. A hyphen is also used to divide a word at the end of a line of type. Hyphens may appear only between syllables. Thus com-pound is properly hyphenated, but compo-und is not.