Let $M_X = (X, d_X)$ and $M_Y = (Y, d_Y)$ each be a D1107: Metric space.
An D4767: Unsigned real number $C \in [0, \infty)$ is a Lipschitz constant for $f$ with respect to $M_X$ and $M_Y$ if and only if \begin{equation} \forall \, x, y \in X : d_Y ( f(x), f(y)) \leq C d_X(x, y) \end{equation}