1660s, "action of enlightening," from enlighten + -ment. Used only in figurative sense, of spiritual enlightenment, etc. Attested from 1865 as a translation of German Aufklärung, a name for the spirit of independent thought and rationalistic system of 18c. Continental philosophers.
For the philosophes, man was not a sinner, at least not by nature; human nature — and this argument was subversive, in fact revolutionary, in their day — is by origin good, or at least neutral. Despite the undeniable power of man's antisocial passions, therefore, the individual may hope for improvement through his own efforts — through education, participation in politics, activity in behalf of reform, but not through prayer. [Peter Gay, "The Enlightenment"]
1530s, "Roman magistrate of 5c. B.C.E. who took censuses and oversaw public manners and morals," from French censor and directly from Latin censor, from censere "to appraise, value, judge," from PIE root *kens- "speak solemnly, proclaim" (source also of Sanskrit amsati "recites, praises," asa "song of praise").
They also had charge of public finances and public works. Transferred sense of "officious judge of morals and conduct" in English is from 1590s. Latin censor also had a transferred sense of "a severe judge; a rigid moralist; a censurer."
From 1640s as "official empowered to examine books, plays (later films, etc.) to see they are free of anything immoral or heretical." By the early decades of the 19c. the meaning of the English word had concentrated into "state agent charged with suppression of speech or published matter deemed politically subversive." Related: Censorial; censorian.