In philosophy (especially Kant), a "thing-in-itself": an object as it is independently of human perception, posited as thinkable yet not knowable through the senses. A noumenon withdraws where a phenomenon appears, meaning's silhouette behind experience.

From Greek nooumenon "that which is thought," from noein "to think," rooted in nous "mind." The irony is deliberate: it names what thought can point toward but cannot empirically seize.