It's about what the class gives access to through inheritance. It has a three letter abbreviation but not API. I've seen this before but can't remember at the moment.
The term is usually used in the context of a framework where you are expected to extend classes from the framework. This would be closely related to the template pattern. However, any class designed for inheritance has this interface. It's the final classes that do not have one.
Put as an SAT question:
Accessible is to API as extendable is to _____
It doesn't seem to be Extension Point. This is the entire set of things that may be overridden though extension on a class. This is something even a simple POJO would have.