Complex sentence: Classes containing pure virtual methods are termed "abstract"; they cannot be instantiated directly, and a subclass of an abstract class can only be instantiated directly, if all inherited pure virtual methods have been implemented by that class or a parent class.
Split into simpler sentences: Classes containing pure virtual methods are termed "abstract"; they cannot be instantiated directly. A subclass of an abstract class can only be instantiated directly if all inherited pure virtual methods have been implemented by that class or a parent class.