Educate me: What the hell is RT60?

I only have one reverb with a control labeled RT60. I just thought it was a fancy or technical name for "decay" or "reverb time" based on what the control did. Looks like I wasn't too far off.
 
In a nutshell: Shoot a start pistol / Clap your hands / Whatever that makes a quick, short and loud transient. RT60 is the time it takes for that sound to fade 60dB.

The principle here is correct for one type of impulse response measurement too. It needs some greatly different parameter changes though to be accurate.

RT60 has been covered above though..