Around 820 AD a Persian guy name Al-Khwarizmi called irrational numbers “inaudible”….. this was later translated to the Latin SURDS (“Deaf” or “mute”). In fact "Surd" used to be considered by another name for "Irrational", but it is now used for a root that is irrational.

Let, ‘x’ be a rational number and ‘n’ be a positive integer such that n√x is irrational, then n√x is called a ‘Surd’ or a ‘Radical’ of order n. 

There are some important notes are given below - 3√11

1)  n√x is a surd only when ‘x’ is rational and n√x is irrational.

2)  When ‘x’ is irrational or n√x is rational, then n√x is not a Surd

Example.1) 3√5, 5√11, 7√8, and 9√14 are surds of order 3, 5, 7, and 9 respectively because 5, 11, 8, and 14 cannot be simplified further

Example.2) 3√64 is not a surd, since 3√64 = 3√43 = 4, is not a surd, since 3√64, which is rational because 64 can be simplified 3√64 = 3√43 = 4.

Example.3)  √π is not a surd, because π is rational.