The word enlightened comes from the Latin prefix 'en' meaning 'in, or into', and the word 'lux' meaning 'light', so : 

In or Into The Light.

In a spiritual context the concept of 'The Light' is often used to signify the Light of 'Divine Truth'.

Words used to describe 'The Light' include : Absolute, Eternal, Infinite - and Unconditional Love.

Using these criteria, Enlightenment would be : the moment of 'Becoming One with The Light'.

It is said...

The moment of Enlightenment is experienced as an irreversible 'flowing-in' of 'The Light'.

The moment when the 'known' reality of the 'self' is instantaneously shown to have been entirely false.

It is known with absolute certainty as the 'completion' of the 'self'.

It is realised that this is the conclusion or destination of the Inner Journey.