A surd is a root such as $$\sqrt{2}$$. They are irrational numbers, which means they cannot be written as either a whole number (integer) or as a fraction. Irrational numbers are decimals which continue forever, without any pattern. A rational number is one which can be written as a fraction. These include all integers, fractions, terminating and recurring decimals.

Two common tasks in surd questions are to simplify a surd and to rationalise the denominator of a surd fraction. To simplify a surd or surd expression, is to make the number in the root sign as small as possible.Rationalising the denominator of a fraction is to eliminate any surds from the denominator.