$h = distance(z, \tilde{z})$, where $\tilde{z}$ is the element that is nearest from $z$ (that is, distance(z, $\tilde{z}$) is smaller than distance(z, any_other_z)).
Is it possible to expression this formally, instead of saying "where $\tilde{z}$ is the element that is nearest from $z$ ..." ?