A nimitta is a sign, that indicates, that the meditator has gained some amount of concentration. Principally is any sign of concentration a nimitta. Hence, there may be just a feeling of stability or calm or lightness, or certain mental factors such as happiness or equanimity, or there may arise some visual perception such as light, colour or some form. And all such occurrences can be called nimittas.

But traditionally there are three kinds of specific nimittas enumerated: First there is the preparatory sign (parikamma nimitta), which is followed by the learning sign (uggaha—nimitta), and at last will arise the counterpart sign (paṭibhāga-nimitta). The preparatory sign is a visual perception of dull, dark colour and fuzzy outline which will usually arise when the mind is trying to attune itself to the physiological processes, especially in the belly. The learning sign is a more clearly outlined perception, at times of some specific colour. This will usually arise when the mind attunes itself to the emotive processes in the heart. And finally there is the sign which fully resembles the mind as such. This may be seen as a perfect image of the idea, person or thing the mind is devoted to.

The quality of any meditation experience will be primarily determined by a) the quality of the concepts used (that is, the ideas that give the incentive for a person to meditate) and b) a persons capacity to make his organism and higher faculties skillfully respond towards those concepts.

These Nimittas are signs, that show that the meditator is starting to transcend the sense-sphere world, at least, that part of it which belongs to ordinary human beings.

Cosmologically any such experience belongs to the experiences of the devas even to those of a lower order, who have not transcended sensuality altogether, but who live in spheres of greater sublimity than human beings do.