A term coined by Ivor Darreg from the Greek words for "strange harmonic". Darreg used it to refer to any non-12-edo (microtonal) scales, which presented strange and wonderful new intervals and sonic worlds to explore; he chose "xen" because its connotation in ancient Greek is that the stranger is to be welcomed as a guest, eventually to become more familiar, and ultimately to become a close friend.
Note that it is possible for a composition to be composed entirely of intervals that would ordinarily be considered 'microtonal' without sounding particularly unusual or 'xenharmonic'; an example is Joe Monzo's 19-tone Samba; other examples would be Renaissance, Baroque and Classical music performed in historically correct temperaments.
An adjective coined by Ivor Darreg from the Greek words "xenos," meaning strange, foreign, or inhospitable, and "harmonikos," (harmonic, etc). to refer to music in non-12-tone equally tempered tuning. The topic noun is xenharmonics.
Xenharmonikôn is a journal devoted to microtonal music which was founded by John Chalmers in 1971.