[bradleyjg]

It is a theorem about the properties of all possible voting systems with preference rankings. The basic idea is there are a few desirable properties for any such voting system, but that it is mathematically impossible for any voting system to satisfy all of them.

[dragonwriter]

> It is a theorem about the properties of all possible voting systems with preference rankings.

More accurately, it is a theorem about the properties of all possible voting systems where the input is voters preference rankings and the output is also a preference ranking.