Bootleg ground

In building wiring installed with separate neutral and protective ground bonding conductors (a TN-S network), a bootleg ground (or a false ground) is a connection between the neutral side of a receptacle or light fixture and the ground lug or enclosure of the wiring device. This connects the neutral side of the receptacle to the casing of an appliance or lamp. It can be a hazard because the neutral wire is a current-carrying conductor, which means the casing can become energized. In addition, a fault condition to a bootleg ground will not trip a GFCI breaker or receptacle that is wired from the load side of a GFCI receptacle. 
Background.
Before 1996, in the United States it was common to ground the frames of 120/240-volt permanently connected appliances (such as a clothes dryer or oven) to neutral conductors. This has been prohibited in new installations since the 1996 National Electrical Code upon local adoption by legislation or regulation. Existing installations are permitted to continue in accordance with NEC 250.140 Exception.
A safer alternative, allowed in recent editions of the National Electrical Code [NEC Sec. 406.4(D)(2)(b)] if a grounding connection is not practicable (where a local electrical code allows it) is to install a GFCI and leave the grounding terminal screw unconnected, then place a label that says "No Equipment Ground" on the GFCI and a marking that states “GFCI Protected” and “No Equipment Ground” on all downstream receptacles.
Correct-polarity bootleg ground.
In the least-dangerous instance of a bootleg ground, a short wire jumper is connected between the bonding screw terminal (usually colored green) on a NEMA 5-15R or 5-20R outlet to the neutral (a.k.a. grounded conductor, colored white according to code) or directly to the white neutral wire via a pigtail. This practice is a NEC code violation, but a standard 3-lamp receptacle tester will report the outlet as correctly wired.
Reverse-polarity bootleg ground.
In the very-dangerous instance of a reverse polarity bootleg ground, the hot and neutral wires have been connected to the opposite terminals, and a jumper or pigtail connection is made between the green bonding screw terminal and what is believed to be the neutral circuit. But because the wiring has been crossed at some point, the hot 120 Volt wire is now connected directly to the ground on the receptacle, placing live voltage on all grounded parts of all equipment plugged into that outlet, thus allowing people to come into contact with a deadly voltage with a current path back to the source (the power transformer) that will not trip either a normal circuit breaker, a GFCI, nor an AFCI soon enough to prevent electrocution.
Other countries.
West Germany banned bootleg grounding in 1973, although it was common practice before and can still be found in older installations.
In Finland using neutral as a ground conductor was a common practice until 1989. After that a ticker PEN-wire was used as both ground and neutral until it was banned in 2007