What is the X in X-mas?
1570s, "fact, occurrence," from Late Latin phænomenon, from Greek phainomenon "that which appears or is seen," noun use of neuter present participle of phainesthai "to appear," passive of phainein (see phantasm). Meaning "extraordinary occurrence" first recorded 1771. Plural is phenomena.
phenomenon phe·nom·e·non (fĭ-nŏm'ə-nŏn', -nən)
n. pl. phe·nom·e·na (-nə)
An occurrence, a circumstance, or a fact that is perceptible by the senses, especially one in relation to a disease.
pl. phenome·nons An unusual, significant, or unaccountable fact or occurrence; a marvel.