What does the term "surd" mean?

The terms "surds" is used when a number is irrational.
Usually, we use this term when we prefer not calculate the square root or the cube root
of a number because the number of decimals is large or
iterative.

For instance, sqrt 2 is a surd but sqrt 9 = 3 is
not.

To add two surds, the radicand must be the
same:

sqrt 2 + sqrt 6

To
multiply two surds is easy if the n-th root of both is the
same.

When the surd is found at denominator of a fraction,
w'e'll have to multiply it by it's conjugate, to change the surd denominator into a
rational number.