What is the definition of a STOL aircraft?

STOL is an acronym for a short takeoff and landing aircraft, which have short runway requirements for takeoff and landing. The Dictionary Of Aeronautical Terms states:

STOL (Short Take Off and Landing). STOL performance of an aircraft is the ability of aircraft to take off and clear a 50-foot obstruction in a distance of 1,500 feet from beginning the takeoff run. It must also be able to stop within 1,500 feet after crossing a 50-foot obstacle on landing.

And according to the FAA:

A STOL aircraft is an aircraft with a certified performance capability to execute approaches along a glideslope of 6 degrees or steeper and to execute missed approaches at a climb gradient sufficient to clear a 15:1 missed approach surface at sea level.