Definition - What does Ati mean?
Ati (also called Atiyoga) is the name of the third of the Inner Tantras within Tantric Buddhism or the Tantric practice of Hinduism. Atiyoga emphasizes the second phase of the complete stage (higher than Anuyoga), at which practitioners achieve the "Great Perfection” through the realization of primordial wisdom or by attaining the natural primordial state. After the attainment of yogic bliss by Anuyoga, Atiyoga is considered the next and highest level of attainment in which individuals realize the true nature of reality through the vehicle of bliss.
In Sanskrit, ati means “beyond” or “over,” and Atiyoga means “excessive union.”
Yogapedia explains Ati
Atiyoga is a philosophy that one's highest perfection or completeness is achieved through insight into the nature of primordial enlightenment, which is free from acceptance and rejection, hope and fear. As the second phase of the complete stage in Tantric practice, it is the point at which one can actualize the immediate causes of enlightenment, as well as the form figures or bodies of Buddha or other divine deities.
Atiyoga emphasizes the natural state, spontaneity and simplicity. It is thought to be a secret teaching or realization that an individual can attain through guided help from a guru. It is believed that upon reaching this “Great Perfection” through intense practice, meditation and spiritual revelation, troubling emotions and thoughts are liberated and one finds that fulfillment is produced without cause within one’s Self.