It’s a class dedicated to a single object. Every object has its own eigenclass — this sounds expensive but in practice they’re lazily created. Regular classes are used to store the definitions of methods which are available on all instances of that class. In contrast, a single object’s eigenclass is used to store the definitions of methods which are available on that object only.

`def foo.bar … end` defines a `bar` method in the eigenclass of the object `foo`, which can then be called with `foo.bar`. (This is how “class methods” work in Ruby: their definitions are stored in the class object’s eigenclass so that those methods can be called only on that specific class rather than all classes.)