The "Enlightenment" is not so much a literary term as a period in history that affected the literature of its time. The Enlightenment can be roughly dated as running from 1660-1789.
The primary change in attitude that marked the period of the Enlightenment was a very conscious effort to rationally evaluate and understand life through logic, scientific investigation, and the application of reason rather than accepting the pronouncements of the Church or tradition as being authoritative explanations. Those involved in advancing the cause of the Enlightened attitudes believed they were advancing to a more enlightened era:
they were leaving behind the dark ignorance and blind belief that characterized the past.
The attitudes of the Enlightenment found their way into many aspects of the culture of the time. It is not coincidence that the Declaration of Independence and the US Constitution were written during this time or that the ending point for the period is the time of the French Revolution.