BSL Maths Glossary - surd - Definition
Subject level: Credit
A surd is the irrational root of a rational number.
If we use the number 4 we know that the square root of that is 2 so that is not a surd.
We can use any number such as 5. the square root af 5 gives us an irrational number so that is also a surd. But if we take the square root of 9 that is 3 which is not an irrational number and therefore is not a surd.