The terms overstable, stable, and understable in disc golf are used to describe the ability of a disc to resist turning in flight.
Overstable:
An overstable disc is one that has a natural tendency to resist turning or curving during its flight. When thrown with sufficient speed, an overstable disc will generally fly on a straight line, with a predictable fade at the end of its flight. Overstable discs are typically recommended for players with more arm speed or those looking for reliable fade and consistency in their throws.
Stable:
A stable disc refers to a disc that flies relatively straight during its flight with minimal turn or fade. It holds its intended line without significant deviation. Stable discs are versatile and suitable for a wide range of players and throwing speeds. They are often favored for accurate and controlled shots, especially when consistency and predictability are desired.
Understable:
An understable disc is one that has a tendency to turn or curve to the right (for right-handed backhand throws) during the initial part of its flight. It offers less resistance to turning and requires less power or speed to achieve its full flight characteristics. Understable discs are beneficial for players with slower arm speeds, beginners, or those seeking increased distance potential and easier turnovers or anhyzer shots.