three turnings of the wheel (Sanskrit, tri-dharma-cakra-pravartana).

1. Notion arising from the Buddha's first sermon where each of the Four Noble Truths is viewed from three perspectives: as a noble truth, as a truth to be realized, and as a truth that has been realized.

2. A Yogācāra theory first taught in the Sandhi-nirmocana Sūtra which classifies the Buddha's teachings into three levels. These are the First Turning which taught the basic Dharma for Śrāvakas (see śrāvakayāna); the Second Turning which taught the provisional doctrine of emptiness (śūnyatā) through such sūtras as the Prajñā-pāramitā Sūtra corpus; and the Third Turning which taught a definitive presentation of the true nature of reality by way of the ‘three natures’ (tri-svabhāva) theory. The content of the Second Turning is implicitly linked to the Madhyamaka school while the Third Turning should be identified with the Yogācāra school, an interpretation naturally not shared by Madhyamaka adherents.

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