surd

In mathematics, numbers often have to be written using roots or fractional exponents. For example, the number $\sqrt{2}$ cannot be written without using a root or writing $2^{1/2}$. A surd is an irrational number which can be written as the sum of rational powers of rational numbers.Another example would be $3\sqrt{2}-\sqrt[3]{5}$.

References

http://www.mathcentre.ac.uk/students.php/all_subjects/arithmetic/surds/resources/34What is a surd?