An irrational number is any number that is not a rational number.
In other words, an irrational number cannot be expressed as a fraction ratio of two integers.
Famous examples of irrational numbers include pi (π), Euler's number (e), and the golden ratio (φ).
For example, pi is often shortened to 3.14159, but is actually an infinite series of numbers. And unlike a rational repeating decimal number such as 0.3333333..., pi does not end with a series of repeating numbers.
Also, almost all square roots of natural numbers are irrational. Only perfect squares are not irrational numbers.
For instance, unlike the perfect square root of 4 (22), the square root of 3 is 1.7320508..., or about 1.73.