We define the order $\nu$ of an extensive map $f$ from a complete lattice to itself: $\nu$ is the smallest of the ordinals a such that $f\circ f^\alpha=f^\alpha$. If $f$ is a preclosure, $f^\nu$ is the closure generated by $f$. We also examine the case of the extensive map which sends an $\wedge$-closed subset $A$ of a complete lattice $L$ towards the $\wedge$-closed subset of the $A$-semi-closed points of $L$.