Augustan, a term deriving from the prestige of Latin literature in the age of Augustus, and 'applied to the period of highest refinement of any national literature' (Oxford English Dictionary), normally refers in England to the years from about 1680 to 1750. It is here stretched from 1660 to 1780 in so far as the outlook of the central period is evident.

From Dryden to Johnson
The Pelican Guide To English Literature, Volume 4
The Social Setting
Note on page 15