The classic definition is the rejection of mysticism and spirituality towards a rational insight.
I'd say it's probably the biggest movement ever, since it more or less built the modern world.
Everything from politics to economics, even technology, and social movements/morals. With no Enlightenment there would still be slavery (even though the Industrial Revolution was the main reason it disappeared).
Yes, quite so, the modern world is built on the Enlightenment. So it is important to understand what it is, so we know what we are defending, when it is attacked by terrorism.
One of the interesting effects of the Enlightenment is counter-intuitive thinking.
For instance, we intuitively know the Sun goes round the Earth, but we counter-intuitively know the Earth goes round the Sun.
And in like manner most of science and mathematics is counter-intuitive.
And indeed modern economics is counter-intuitive for we intuitively know greed (called usury) is bad, but we know since Adam Smith counter-intuitively that private greed gives public prosperity, under regulated and competitive capitalism.
And every politician intuitively wants to maximise their power, but we know counter-intuitively that the limitation of power is the heart and soul of liberal democracy.
At a deeper level we know children learn to speak their native tongue intuitively at home, but counter-intuitively children are compelled learn to read and write at a special institution by State Law.
So children naturally learn to speak at home but are taken out of their warm homes and taught the unnatural and counter-intuitive task of reading and writing.
And of course almost all religion is intuitive, as is astrology and mbti.
But as I write the world is changing and children across the world learn, naturally and intuitively, to use the telephone, radio, the television, and the computer in the comfort of their own homes, without any need to be compelled by the State to go to a cold institution to learn to use these electronic media.
So the Enlightenment gave us the counter-intuitive, and the electric media are returning us to the intuitive.