Here's a definition I follow. 1 hour and not to utter exhaustion.

What is functional threshold power?

Functional threshold power (FTP) is the maximum power output an athlete can maintain in a quasi-steady state without fatiguing for 1 hour. FTP is, in my opinion, is a better way to analyze current fitness and improvements because it is more readily available to the athlete, isn’t invasive, and can be repeated multiple times for FREE! More about FTP here.

How did FTP and LT become confused to mean the same thing? Because your maximum effort for 1 hour is commonly the same as your lactate threshold. Dr. Andy Coggan realized this so he came up with the FTP concept and simplified things for everyone (yippee!). However, your FTP is not your LT and your LT is not your FTP, got it!?

So, to recap: what is the difference between lactate threshold and functional threshold power? LT is the point at which lactate increases in the blood stream exponentially whereas FTP is the maximum effort an athlete can maintain for 1 hour without fatiguing