Definition of: noumenon

(n′mə·non, nou′-) noun plural ·me·na (-mə·nə) Philos.
1. An object of intuitior by the reason or understanding, as something transcending perception through the senses. opposed to phenomenon.
2. The unknown ground or cause of phenomena, regarded as necessarily assumed by the mind, but the real nature of which is wholly transcendent; the unknowable thing in itself. [<NL <Gk., orig. ppr. passive of noeein think]