A surd is an irrational number in root form.A number that cannot be expressed as a simple fraction i.e.a/b You can think of surds as being square roots of numbers that do not have a whole number as the root. Surds crop up in many situations in mathematics and we will learn some basic rules to deal with them. When we can’t simplify a number to remove a square root (or cube root etc) then it is a surd. Example: √4 (square root of 4) can be simplified (to 2), so it is not a surd! The surds have a decimal which goes on forever without repeating: they are actually Irrational Numbers.