Per day. When the winds or temperature change, it becomes an entirely new object. Instead of landing one way, you may choose to land in another direction, which completely changes the whole jump.
Also, if you've had lunch and are loading your canopy more heavily from one jump to the next, then the 25-jump-per-day clock should be reset.
Furthermore, with a sufficient amount of "Woohoo's" and "Yea's", any jumper may be exempted from this limit