What's a "Fence?"
 

I have heard the term "fence" used when describing (older) shotguns, but I am confused as to its meaning. Can anyone help?

Ahhh I believe that the fence is the raised area behind the barrels on the action often engraved.

The standing breech through which the firing pins protrude and as stated above.

Thanks both of you ... and by the way, you are both correct. Here is another definition of that I have found since posting my question:

Hemispherical outgrowths of the receiver of a double gun that mate with the breech ends of the barrels. The term derives from the flanges (or fences) in this position on a muzzle loading gun that were designed to protect the eyes of the shooter from sparks and escaping gasses.