What is the X in X-mas?
late 14c., from Old French chanter "to sing, celebrate" (12c.), from Latin cantare "to sing," originally frequentative of canere "sing" (which it replaced), from PIE root *kan- "to sing" (cf. Greek eikanos "cock," Old English hana "cock," both literally "bird who sings for sunrise;" Old Irish caniaid "sings," Welsh canu "sing"). The frequentative quality of the word was no longer felt in Latin, and by the time French emerged the word had entirely displaced canere. Related: Chanted; chanting.
1670s, from chant (v.), or else from French chant (12c.), from Latin cantus "song, a singing; bird-song," from past participle stem of canere.
the Gregorian chant (q.v.) and, by extension, other similar religious chants. The word derives from the 13th-century Latin term cantus planus ("plain song"), referring to the unmeasured rhythm and monophony (single line of melody) of Gregorian chant, as distinguished from the measured rhythm of polyphonic (multipart) music, called cantus mensuratus, or cantus figuratus ("measured," or "figured," song). Its other main application is to ancient Christian music with the same unmeasured rhythm and monophony-in the West, Ambrosian, Gallican, and Mozarabic chants (qq.v.); in the East, Byzantine, Syrian, Coptic, Ethiopian, and Armenian chants (qq.v.). It may also refer to similar non-Christian religious music, such as Jewish and Hindu chants.