Irrational number

An irrational number is a number that can't be written as a simple fraction. When you write a number as a fraction, it has two parts - the numerator (the top number) and the denominator (the bottom number). If both of these parts are whole numbers (e.g. 3/4), then it's a rational number. But if either the numerator or the denominator (or both!) isn't a whole number (so it can't be written as a simple fraction), then it's an irrational number.