666 (number)

666 is a natural number after 665 and before 667. It is a number with many properties. 
One of these properties is that it is the sum of the first 36 natural numbers. That makes 666 a triangular number. 
666 is the sum of the first 144 decimals (digits after the decimal point) of pi. The number is also used in the "666 dating rule" or "rule of sixes" which judges the attractiveness of men.
In religion.
The Bible says in the Book of Revelation that the number 666 is often associated with an Antichrist. It is believed that the number is the "Number of the Beast". Some interpreters say that a future world leader will force people to accept this number as a mark or else be forbidden to buy or sell. 
However, many people who do accept this mark will be rejected by God. However, those who believe in numerology or superstitions believe it is an angel number associated with the spiritual imbalance and do not necessarily represent evil or bad luck. 
Another interpretation notes that in the Aramaic alphabet each letter has a numeric value, the letters in each name or title can be added to produce a total. 
Some think the name of the Emperor Nero was secretly referred to by this number, since Nero did much to harm Christians. The name of Nero Caesar in Aramaic adds up to 666. 
In pop culture.
There were many movies about the number 666, such as: