\begin{definition}[Definition:Diophantine m-Tuple/Quadruple]
A '''Diophantine quadruple''' is a Diophantine $m$-tuple where $m = 4$.
That is, a '''Diophantine quadruple''' is a set of $4$ positive integers such that the product of any $2$ of them plus $1$ is a square number.
\end{definition}