Bootleg Ground or "False Ground"
A bootleg ground is a connection between the neutral and grounding lugs of a receptacle. It's typically done to give the impression that a 3-prong receptacle is grounded when actually there is no equipment grounding conductor installed.
It's dangerous because the neutral conductor carries current. If it's connected to the receptacle grounding lug and an appliance with a grounding prong is plugged in, current can then flow onto the appliance and if a person in contact with that appliance is grounded, they'll become part of the circuit, resulting in electric shock or electrocution.
Although they work fine for identifying common receptacle miswiring problems, bootleg grounds will fool a 3-light tester. Some circuit analyzers claim to be able to identify them, but many produce false positives.