An irrational number is a number which can't be expressed as a simple fraction, like 1.23. In arithmetic, these numbers are also commonly called 'repeating' numbers after division, like 3.33 repeating, as a result of dividing 10 by 3. Although irrational numbers aren't entirely efficient, they're sufficiently accurate for some uses, like basic measurements. New mathematical methods are progressively removing these issues with irrational numbers.