Absolute Truth refers to a reality which doesn't change over time. Absolute means all other truths are relative to it, or depend on it. When we talk about God, or Krishna, we're talking about the Absolute Truth.
The Absolute Truth is the source of everything, the ultimate cause of all causes. In Sanskrit, it is also called satyam param, the highest truth.
This supreme truth can be perceived in three features—as Brahman, all-pervading, impersonal oneness, as Paramatma, the manifestation of God within the heart of every being, and as Bhagavan, the Supreme Personality of Godhead. These three are the same one truth, understood from increasingly advanced levels of realization.
In the beginning the Absolute appears impersonal. With more advancement, the Supreme is perceived within one's own heart and the hearts of others. The highest realization is to see the Supreme Truth as the Supreme Person, who is complete in wealth, power, fame, beauty, knowledge, and renunciation. The Bhagavad-gita, the Srimad-Bhagavatam, and other Vedic literatures identify this Supreme Personality of Godhead as Krishna.
Courtesy of Back to Godhead Magazine's Yoga Dictionary, #15-12, 1980