In the SmalltalkLanguage, a method selector is more commonly known as a MessageSelector.  A message sent to an object (the receiver) contains a selector.  The receiver looks up the first ClassMethod in its ClassHierarchy that matches the selector, and evaluates that method, returning the result to the CurrentContext.

Other ObjectOriented languages have closely related semantics.  

The process of looking up an appropriate method is known as MessageDispatch or MethodDispatch.