1570s, "a fact directly observed, a thing that appears or is perceived, an occurrence," especially a regular kind of fact observed on certain kinds of occasions, 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 "bring to light, cause to appear, show" (from PIE root *bha- (1) "to shine"). Meaning "extraordinary occurrence" is recorded by 1771. In philosophy, "an appearance or immediate object of experience" (1788). The plural is phenomena.
<a href="https://www.etymonline.com/word/phenom">Etymology of phenom by etymonline</a>
Harper, D. (n.d.). Etymology of phenom. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/phenom