OOP concept: Abstract describes something
that is declared but not fully defined. For example, an abstract
method is a method that is declared as a part of a class, but has no
implementation. It cannot be called, it is only meant to be
inherited by derived classes.
SIDL keyword: Abstract is an optional modifier for both classes and methods. An abstract method is a method that has no implementation, it's a way of declaring a method that every subclass must implement for itself. An abstract class has one or more abstract methods, and therefore cannot be instantiated.