Definition:Radicand

Definition
Let $\sqrt[n] P$ denote the $n$th root of an expression.

Then $P$ is the radicand of $\sqrt[n] P$.

That is, it is the object whose $n$th root is to be taken.

Linguistic Note
The word radicand comes from:
 * the word radical, itself deriving from the Latin radix meaning root
 * the suffix -and, originating from the Latin gerundive form, loosely meaning that which needs to be ...

That is, radicand literally means that which needs to have its root extracted.