Fiat Lux means "Let there be light", or "Let there be light". It is a biblical term, used by God during the creation of the universe, when he created light, out of nothing. It's a well-known metaphor, and it represents the origin of the new.
The phrase comes from the third verse of the Book of Genesis, when it says, "In the beginning God created heaven and earth. And the earth was formless and void; and there was darkness over the face of the deep. and the Spirit of God moved over the face of the waters. And God said, Let there be light: and there was light. And God saw the light, and it was good, and God separated the light from the darkness."
When you create an idea, a thought, we are simply creating something new, that is, "Fiat Lux". The phrase is also used for another meaning of the metaphor, that of dispelling ignorance.
Fiat Lux in Freemasonry
Fiat Lux is also a term widely used in Freemasonry, it is said to be the culmination of initiation, at which time the candidate receives the light of Masonic wisdom, since he was in the darkness of the world profane.
Know the meaning of other phrases in latin.
