Which of the following is an irrational number?

Irrational numbers are defined as numbers that cannot be expressed as the ratio of two integers. This means that their decimal expansions are non-terminating and non-repeating. Among the given choices, π (pi) is the only number that fits this definition. It is approximately equal to 3.14159, and its decimal representation goes on infinitely without repeating.

The other choices represent rational numbers. The number 2 is an integer and can be expressed as the fraction 2/1. The square root of 4 is 2, which is also rational. Lastly, 0.5 can be expressed as the fraction 1/2, making it rational as well. Thus, π stands out as the only option that is irrational.