a fact or situation that is observed to exist or happen, especially one whose cause or explanation is in question. 5 days ago · phenomenon noun [C] (EXISTING THING) · paranormal phenomena · geophysical and geological phenomena · Johnson (op. A phenomenon is "an observable fact or event. a fact, occurrence, or circumstance observed or observable: to study the phenomena of nature. Phenomenon definition is - an observable fact or event. A phenomenon is "an observable fact or event.