Leak

A leak is when a particular event happens to make a character or object move so fast that it goes outside a particular border of the map and fly outside the map's boundaries.

Leaks used for cheating
In the popular Halo: Combat Evolved map Danger Canyon, players have used leaking to their cheating advantage when Team Kill is turned off.

This is useful in the particular map because you have to go around in a circle to get to the other base, when geographically the other base is right next to yours, only a wall separating the two bases.

An example of this is when you get someone from your team to run you over with a ghost or warthog into the wall between the two bases. If you get hit fast enough, you will fly through the wall, and land in the other base. That'll almost certainly get you banned or kicked.

Other causes of leaking
Another cause of leaking is when an object collides with you, increasing your velocity so much that you go very far, or fly out of the map. One example of this, is if you use modding to change the "Vehicle Type" flag of the Warthog to a "Banshee" or "Alien Fighter". The moment you get in the warthog and move it, you will be flung in the air, only to die moments later. This is to do with the physics engine of Halo.