Pythagoras was an ancient Greek philosopher and mathematician who is best known for the Pythagorean theorem, which states that in a right-angled triangle, the square of the length of the hypotenuse (the side opposite the right angle) is equal to the sum of the squares of the lengths of the other two sides. He is also credited with the discovery of the five regular solids, and he founded the Pythagorean Brotherhood, a religious and philosophical community that had a significant influence on Western thought. Pythagoras lived in the 6th century BC and is considered one of the most important figures in the history of mathematics and philosophy.