The Nil Object

In the SmalltalkLanguage, "nil" is used to represent "no object."

It is...

A global symbol that references a unique object that represents "no object."
The "nil" instance is unique -- SingletonPattern.
The "nil" object is an instance of the class UndefinedObject. So, if you send it messages that are defined on the UndefinedObject class, it will respond accordingly.
Returns true for the "isNil" message. (All other classes return false.)

Class Object defines an "isNil" method that answers (returns) false. UndefinedObject defined an "isNil" method that answers true.