Let $M = (X, d)$ be a D1107: Metric space.
A D62: Sequence $x : \mathbb{N} \to X$ is a Cauchy sequence with respect to $M$ if and only if
\begin{equation}
\forall \, \varepsilon > 0 : \exists \, N \in \mathbb{N} \, (n, m \geq N \quad \implies \quad d(x_n, x_m) < \varepsilon)
\end{equation}