Traditionally, 2000 feet of separation was required for aircraft operating above FL 290. Due to the thinner air, altimeters and autopilots used were not able to maintain altitude keeping well enough for 1,000 feet of separation to be considered safe at these high altitudes.
Improved equipment now allows that 1,000 feet of vertical separation to be extended all the way up to FL 410. This reduced vertical separation increases the capacity and efficiency of the air traffic control system. However, only approved aircraft may utilize these altitudes.
The altitudes between FL 290 and FL 410 are referred to as RVSM, which stands for reduced vertical separation minimums. Aircraft that are not specifically approved for flight into RVSM airspace are referred to as non-RVSM.
Reference: AIM 4-6
Reference: FAR 91.180