An abstract class is a class that cannot be directly instantiated. An instance can be created only in conjunction with an instance of one of its descendant subclasses. 
An action is an individual operation that performs a single task on an element of a model. 
An activation is the execution of a single procedure in a state machine instance at some time. 
An actor is a role played by an external entity that requires something from the system in one or more interactions with the system. 
An archetype is a fragment of data access and text manipulation logic that states formally how to embed an Executable UML model into text. 
An assigner is a state machine that serves ...