A short definition of Illumination by Roberto Assagioli:
One of the ways in which the superconscious manifests most frequently in the consciousness is illumination, which follows the opening of the “inner eye”. Although intuition and illumination have a close affinity, each presents points of difference. In a general way an intuition can be said to be an illuminative flash concerning some particular aspect or manifestation of Reality. But illumination is something more expansive and enduring; it is a vision that reveals the essential nature and synthetic unity of the whole of Reality, or of some of its major aspects. It is the perception of a “light” which, emanating from Reality itself, is different from physical light. Much evidential testimony relating to this experience is contained in William James’ Varieties of Religious Experience and Winslow Hall’s Observed Illuminates. This type of illumination may be regarded as the revelation of divine immanence, of the unity of the Universal Life as it manifests through myriads of forms. Its most pertinent description is that contained in Bock X of the Bhagavada Gita, in which it is termed the “Revelation of the Universal Form”.
A number of poets have had and tried to express this experience of illumination, the greatest of them being Dante, who fills the “Paradise” of his Divine Comedy with expressions of light. At the beginning of the Canto he clearly claims to have undergone the ineffable experience of the loftiest Light, the light that pervades the highest level, the “heaven” closest to the Supreme Reality which is God.
In the consciousness of its percipients, manifestation of the light assumes different aspects; or rather, different aspects of light dominate according to the individual, since they are not separate but interpenetrate and fuse in varying proportions. Sometimes, as in the case of Tagore, the cognitive aspect is overriding. In the Christian as well as the Eastern mystics it is associated with feelings of love and adoration. In others it arouses a sense of joy, which may reach ecstatic bliss. But, I repeat, it is a matter of the prevalence of one or other of these aspects. In general all are present to a certain extent. From Transpersonal Inspiration