The acronym for Variable Specific Impulse Magnetoplasma Rocket.
The Vasimir is an experimental rocket engine design being developed by NASA to facilitate travel to Mars. This rocket design would allow for constant thrust during space flight, as opposed to the current engines which only allow for a few short thrusts. By using a constant thrust rocket, a ship would be able to achieve speeds magnatudes faster than current ships, cutting flight time for a trip greatly.
The fuel used would be light; hydrogen or helium, allowing enough to be stored in a ship for a trip to the furthest reaches of our solar system. In order to be able to use these types of fuel, the engine must be able to achieve temperatures in the neighborhood of 1 million degrees Celcius, which is hot enough to melt all engine components. To contain the super heated fuel (which at that temperature exists only as plasma) the engine uses multiple magnetic containment fields.