Critical Power is the power an athlete can sustain for a very long time, without fatigue, in theory. It is calculated using a model and time to exhaustion tests of 2 and 12 or 20 minutes (see also FTP)

In practice, CP and FTP are usually close to each other. But since it is very difficult for an athlete to maintain the desired intensity FTP ‘reality’ tends to result in a number that is much lower than CP ‘theory’. Often users will consider the two as the same.