Maha Ati is one sub-division of the nine yanas taught by the Nyingma school of Tibetan Buddhism. The highest three yanas, or vehicles, in this tradition are the three inner tantras, Maha Yoga, Anu Yoga, and Ati Yoga. Nyingma masters have further divided each of these three into three further divisions. Thus, the first inner tantra, Maha Yoga, is sub-divided into Maha-Maha, Maha-Anu, and Maha-Ati, thus making Maha-Ati the third sub-division of nine.
This term was also used by Buddhist teacher Chögyam Trungpa Rinpoche as a Sanskrit equivalent of the Tibet term Dzogpa chenpo, although this back-translation is unconventional and did not catch on more widely.