The noumenon (plural: noumena) classically refers to an object of human inquiry, understanding or cognition. The term is generally used in contrast with, or in relation to, "phenomenon" (plural: phenomena), which refers to appearances, or objects of the senses. A phenomenon is that which is perceived; A noumenon is the actual object that emits the phenomenon in question.
The philosopher Immanuel Kant used the term noumenon synonymously with the phrase thing in itself (German: Ding an sich).
Reference:
http://en.wikipedia.org/wiki/Noumena
