gh-161007: Add a glossary entry for "virtual subclass"

Author: thejesh23

Doc/glossary.rst

@@ -29,9 +29,10 @@ Glossary
       Abstract base classes complement :term:`duck-typing` by
       providing a way to define interfaces when other techniques like
       :func:`hasattr` would be clumsy or subtly wrong (for example with
-      :ref:`magic methods <special-lookup>`).  ABCs introduce virtual
-      subclasses, which are classes that don't inherit from a class but are
-      still recognized by :func:`isinstance` and :func:`issubclass`; see the
+      :ref:`magic methods <special-lookup>`).  ABCs introduce
+      :term:`virtual subclasses <virtual subclass>`, which are classes that
+      don't inherit from a class but are still recognized by
+      :func:`isinstance` and :func:`issubclass`; see the
       :mod:`abc` module documentation.  Python comes with many built-in ABCs for
       data structures (in the :mod:`collections.abc` module), numbers (in the
       :mod:`numbers` module), streams (in the :mod:`io` module), import finders
@@ -1727,6 +1728,15 @@ Glossary
       A computer defined entirely in software.  Python's virtual machine
       executes the :term:`bytecode` emitted by the bytecode compiler.
 
+   virtual subclass
+      A class that :func:`issubclass` and :func:`isinstance` treat as a
+      subclass of an :term:`abstract base class` even though it does not
+      inherit from it.  A class becomes a virtual subclass when it is
+      registered with the ABC's :meth:`~abc.ABCMeta.register` method, or when
+      the ABC's :meth:`~abc.ABCMeta.__subclasshook__` recognizes it.  The ABC
+      does not appear in the virtual subclass's :term:`MRO`, and none of the
+      ABC's methods are inherited.
+
    walrus operator
       A light-hearted way to refer to the :ref:`assignment expression
       <assignment-expressions>` operator ``:=`` because it looks a bit like a