Which is an irrational number?

An irrational number is a number that cannot be expressed as a simple fraction - that is, the ratio of two integers. It's a real number that has a non-repeating, non-terminating decimal expansion. This means that when you try to write it down in decimal form, the digits go on forever without any repeating pattern.

One of the most well-known irrational numbers is Pi (π), which is the ratio of the circumference of a circle to its diameter and is approximately equal to 3.14159... The digits after the decimal point go on forever without repeating.

Another example is the square root of 2 (√2), which is approximately equal to 1.41421356... Like π, its decimal expansion goes on forever without repeating, and it cannot be precisely written as a fraction.

Other famous irrational numbers include the mathematical constant e (which stands for Euler's number and is approximately equal to 2.718281828459...), and the golden ratio (ϕ, which is approximately equal to 1.618033988749895...).

All these numbers are examples of irrational numbers because they cannot be expressed exactly as a ratio of two integers.