Numbers that cannot be expressed as an exact ratio are called irrational numbers; numbers that can be expressed as an exact ratio are called rational numbers. For instance, 1/2 (one half, or 50 percent of something) is rational; however, 1.61803 (π), 3.14159 (p), 1.41421 (√2), are irrational. History claims that Pythagoras in the sixth centuryB.C.E. first used the term when he discovered that the square root of 2 could not be expressed as a fraction.