tantra
Through the practice of tantra one is seeking to achieve the supreme feat of Buddhahood in order to become a source of help and happiness for all beings. Along the way common feats are also sought for the sake of enhancing the accumulation of merit necessary for attaining Buddhahood. For both supreme and common feats deity yoga is necessary, the initial process being called 'approximation' because through imagining the deity one is approaching closer to it. Without preliminary approximation the feat of lengthening the lifespan, becoming youthful, gaining the five clairvoyances, and so forth cannot be achieved. It is to achieve such feats that prior approximation is performed, called 'prior' because it necessarily precedes actualising a specific feat and using it for the welfare of others.
--from Deity Yoga in Action and Performance Tantra by His Holiness the Dalai Lama, Tsong-ka-pa, and Jeffrey Hopkins, published by Snow Lion Publications | http://ub0.cc/1v/4L
