[marcandre]

In Ruby they are called "singleton classes", but the actual technical term is metaclass: https://en.wikipedia.org/wiki/Metaclass (check the Ruby entry)

[masklinn]

> the actual technical term is metaclass

Hard disagree. metaclass normally is the class of a class (aka `Class` in ruby), and is used to work manipulate classes.

In Ruby, a metaclass is instead an implicit per-object class.