phenomenal adj.

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.

suffix forming adjectives from nouns or other adjectives, "of, like, related to, pertaining to," Middle English -al, -el, from French or directly from Latin -alis (see -al (2)).

"philosophical doctrine or way of thinking which holds that phenomena are the only realities or objects of knowledge," 1856, in a Christian context (opposed to materialism), from phenomenal + -ism. Used earlier in the same sense was phenomenism (1830). Related: Phenomenalist (1856).

I AM about to try to explain a manner of thought which, in various applications, or perhaps misapplications, of it, I have been in the habit of mentally characterizing, and perhaps of speaking of, as 'positivism.' I shall now however not use this term, but the term 'phenomenalism.' I understand the two terms to express in substance the same thing .... The reason for the change is, because in the purely intellectual application which I shall now make of the term, ''phenomenalism' may perhaps carry with it less danger of extraneous associations being joined with it, and may express what I mean more generally .... [John Grote, "Rough Notes on Modern Intellectual Science," 1865]