Noumenon (νοούμενoν) is a Greek word that could be translated as "something that is thought", or "the object of an act of thought". This word is sometimes used for object or principles that cannot be known through the use of the senses.
1. The intellectual conception of a thing as it is in itself, not as it is known through perception.
2. The of itself unknown and unknowable rational object, or thing in itself, which is distinguished from the phenomenon through which it is apprehended by the senses, and by which it is interpreted and understood; -- so used in the philosophy of Kant and his followers. 
The term is generally used in contrast with, or in relation to "phenomenon", which refers to anything that appears to, or is an object of the senses.
In Plato's philosophy the noumenal realm was equated with the world of ideas known to the philosophical mind, in contrast to the phenomenal realm, which was equated with the world of sensory reality:
Platonist frames of thought draw a dividing line between two realms. One realm, the inferior of the two, is the material, physical world of sense experience. It is the "phenomenal" world, the world of objects, of the body, of immediate perception. The other, superior realm is the world of the immaterial, the spiritual, the world of realities not accessible to the body's senses, the world known by intellect or spiritual sense, the "noumenal" world.
The term noumenon in the Theosophical literature is generally used to refer to "The true essential nature of being as distinguished from the illusive objects of sense." In the septenary model of the constitution of the universe, the three higher planes are normally regarded as "noumenal".
Phenomenon (Greek: φαινόμενoν, phainomenon, from the verb φαίνειν, phainein, to show, shine, appear, to be manifest or manifest itself, plural phenomena), is any thing which manifests itself. Phenomena are often, but not always, understood as "things that appear" or "experiences" for a sentient being, or in principle may be so.
The term came into its modern philosophical usage through Immanuel Kant, who contrasted it with the noumenon. In contrast to a phenomenon, a noumenon is not directly accessible to observation. Kant was heavily influenced by Leibniz in this part of his philosophy, in which phenomenon and noumenon serve as interrelated technical terms.
- Noumenon at Theosopedia