From 3. Data model:
Instance methods
An instance method object combines a class, a class instance and any callable object (normally a user-defined function).
If it is a definition, what does it mean?
If it is not a definition, what is the definition of an "instance method"?
Is an "instance method" the same concept of a method of a class?
Since someone brings up class methods and static methods, bound methods and unbound methods, let me clarify:
I understand a method of a class can be an ordinary method, a class method, or a static method. I understand a method of a class accessed via the class or its instance can be bound or function. I have never heard of "an instance method". I don't know what it is even after looking at the quote and am not sure if it is related to a ordinary method, a class method, or a static method, or a bound method or function.
Your confusion comes from what exactly this definition is about. The term "instance method" is actually used to describe both the concept (a method that works on an instance - by opposition with a classmethod or staticmethod) and its technical implementation. The definition you quote is about the technical implementation.
If you want to understand the context of this definition, you can read this article in the Python wiki, which explains how Python turns functions into methods at runtime.