gradual enlightenment 

In Chinese Ch'an history, this represents the view, traditionally imputed to the ‘Northern School’, that enlightenment (bodhi) is acquired gradually through long practice. A famous verse that appears in The Platform Sūtra of the Sixth Patriarch attributed to Shen-hsiu (606-706) compares the mind to a clear mirror upon which dust has accumulated. By wiping away the dust, the mirror gradually emerges in its clarity. Subsequent Ch'an thought came to reject this conception of religious practice, claiming that it was based on a false duality between delusion and enlightenment, practice and attainment, and prior and subsequent moments in time. In its place, Ch'an came to embrace the view of ‘sudden enlightenment’, which held that the mind was always inherently clear and enlightened, and that beings had only to realize this fact, a feat that could be accomplished instantaneously.

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