A suspended electrified wire which is used to deter (kill) people or cows from moving from one side to the other.
The trick to surviving an encounter with an electric fence is to not grab onto it to see if it is live, but rather to flick it with your fingernail, and then if you can't feel anything, rest your fingernail on the fence, with your hand open, palm up. For a few seconds.
Now look down. If your hand is open, then it's probably safe to assume the fence is switched off.
However, if your palm is closed and there's blood seeping out of the puncture wounds where your blackened, smoking nails have driven themselves into your hand, then you should probably consider a different path.