Enlightenment

Enlightenment is a fundamental philosophical concept which grew beyond religion and spirituality and essentially means being illuminated by acquiring new wisdom or understanding.

While "enlightenment" mainly refers to the secular European Age of Enlighenment, it also has some parallels in both Far Eastern religious concepts (the Buddhist Bodhi, the Zen Satori, and the Hindu moksha) as well as the European and Near Eastern religions (ie. Judaism, Christianity and Islam).

Kant's Definition of Enlightenment 
(Note: Kant is here talking about "Aufklärung", a phase in cultural history marked by a faith in reason; this is quite different from "Erleuchtung", the spiritual awakening spoken of above in relation to e.g. the Buddha. Only in English is the same word used for these two different phenomena.)

In his famous 1784 essay What is Enlightenment?, Immanuel Kant described it as follows:

Enlightenment is man's release from his self-incurred tutelage. Tutelage is the incapacity to use one's own understanding without the guidance of another. Such tutelage is self-imposed if its cause is not lack of intelligence, but rather a lack of determination and courage to use one's intelligence without being guided by another.

Kant reasoned that although a man must obey in his civil duties, he must make public his use of reason. His motto for enlightenment is Sapere aude! or "Dare to know."

Adorno's and Horkheimer's Definition of enlightenment

In their controversial analysis of the contemporary western society, Dialectic of Enlightenment (1947), Theodor Adorno and Max Horkheimer developed a wider, and more pessimistic concept of enlightenment. In their analysis enlightenment had its dark side: while trying to abolish superstition and myths by 'foundationalist' philosophy, it ignored its own 'mythical' basis. Its strivings towards totality and certainty led to an increasing instrumentalization of reason and thus it was ultimately responsible for the horrors of the Nazi Holocaust. In their view the enlightenment itself should be enlightened and not posed as a 'myth-free' view of the world.