Pythagoras was an ancient Greek philosopher and mathematician, best known for his contributions to mathematics, particularly the Pythagorean theorem, which relates the sides of a right triangle. He founded a religious movement known as Pythagoreanism, which emphasized the importance of numbers and their relationships to the cosmos, ethics, and the soul. Pythagoras also believed in the concept of reincarnation and the immortality of the soul. His teachings influenced various fields, including mathematics, philosophy, and Music Theory.