Noumenon
Etymology
From Greek "nooumenon," that which is thought or apprehended by the mind, from "nous," mind. Kant paired it against the "phenomenon," the thing as it appears, arguing that we can know only appearances and never the noumenon standing behind them.
noun · /NOO-muh-non/
A thing as it is in itself, independent of how it appears to us — and, for Kant, unknowable.
What it names
The reality behind appearances, the "thing in itself" our senses can never actually reach.
Example
The physicist admitted her equations described how matter behaves, not the noumenon — what matter ultimately is.
Related idea
The shadow-twin of the phenomenon in phenomenology, and built from the same "nous" as the intellect; the noumenon is the one thing perception can never deliver.
See also