A permanent displacement of part of the O-ring into the small clearance between the seal and shaft as a result of the action of a high fluid pressure.

If you have a high-pressure application you will need a back-up ring installed in the O-ring groove to prevent the extrusion.

Here is a picture of the back up ring. You can see it cross-hatched in the illustration.

The back up ring is often made of Teflon® and does a good job of preventing O-ring extrusion in higher pressure applications.