\begin{definition}[Definition:Multiplicity (Polynomial)]
Let $R$ be a commutative ring with unity.
Let $P \in R \left[{X}\right]$ be a nonzero polynomial.
Let $a \in R$ be a root of $P$.
The '''multiplicity''' of $a$ in $P$ is the largest positive integer $n$ such that $\left({X - a}\right)^n$ divides $P \left({X}\right)$ in $R \left[{X}\right]$.
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A '''double root''' is a root of multiplicity at least $2$.
Category:Definitions/Polynomial Theory
\end{definition}