Towards the end of this article there is a reference to the 'metaclasss'. It should probably be noted that the prefered terminology in the Ruby community now is 'singleton class' and not 'metaclass'.

For quite a while there was little agreement on this name because there was no standard method that could be called to return the singleton class and therefore no agreed upon name. The only access was via syntax:

  (class <<object; self; end)

The results of this expression were called metaclass, eigenclass, singleton class, and probably a few more names.

There is now a standard method:

  object.singleton_class

and so, for the most part, the naming confusion is fading away.