atiyoga (Sanskrit). The sixth level of tantric teachings according to the Nyingma school, equivalent in some respects to the Non-dual Tantra class of the New Schools. The tantric texts that form this class are unique to the Nyingma school. The term is often used as a synonym for Dzogchen. The atiyoga class of teachings is further subdivided in the three groups: the Mind Category, the Spatial Category and the Secret Instruction Category.

A Dictionary of Buddhism, Oxford University Press, 2003, 2004 (which is available in electronic version from answer.com)