If you take it in math's sense then it is number thats irrational or the irrational root of an integer ............
n if you wanna take it in normal english then it is a sound, a type of sound spoken using breath and not using your sound (voice) ........

Hope it helps !!
Plss mark it as the best if it helps .............. plss
Surds. When we can't simplify a number to remove a square root (or cube root etc) then it is a surd. Example: √2 (square root of 2) can't be simplified further so it is a surd. Example: √4 (square root of 4) can be simplified (to 2), so it is not a surd!

In general, an unresolved nth root, commonly involving a radical symbol , is known as a surd. However, the term surd or "surd expression" (e.g., Hardy 1967, p. 25) can also be used to mean a sum of one or more irrational roots. In the mathematical literature, the term arises most commonly in the context of quadratic surds.

The term "surd" has a special meaning in the Wolfram Language, where the principal th root of a complex number  can be found as z^(1/n) or equivalently Power[z, 1/n]. However, when  is real and only real roots are of interest, the command Surd[x, n] which returns the real-valued th root for real  odd  and the principal th root for nonnegative real  and even  can be used.

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