In later Advaita, as we have noted earlier, a distinction is made between two types or kinds of spiritual experience related to transcendental consciousness: that of savikalpa samadhi, the borderline experience, as it were [...] and that of nirvikalpa samadhi, the pure experience of Reality.
Savikalpa samadhi means "determinate" spiritual experience. In it an awareness of duality is absent, but unlike susupti, the deep-sleep state, the emphasis here is not so much on the absence of duality as it is on the presence of non-duality. Whereas in susupti the self is still a knowing subject, although there is nothing there as such to be known, in savikalpa samadhi the self is aware only of the presence of Reality. In susupti all the phenomenal activities self are suspended in a kind of serene blank; in savikalpa samadhi they are concentrated on the Real. In susupti the self is still very much the jiva; in savikalpa samadhi the self is passing into the Atman.
Nirvikalpa samadhi is the consumation of the process. It is the pure "indeterminate" intuition of non-duality. In savikalpa samadhi the Self is aware of Reality; in nirvikalpa samadhi it is Reality.
Source: Advaita Vedanta : A Philosophical Reconstruction, Pages: 62,63