To those not familer with OO-jargon, an abstract class is where the methods of a class are defined but not implemented.
That's not the generally agreed-on definition.
As a source on this (a source I have within reach), I'll cite the "Gang of Four" Design Patterns book, page 15:
An abstract class is one whose main purpose is to
define a common interface for its subclasses.
An abstract class will defer some or all of its
implementation to operations defined in subclasses;
hence an abstract class cannot be instantiated.
That's the definition I've been used to working with for the past decade or so.
You might be thinking of an "interface".
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