In the Unified Modeling Language, an action[1] is a named element that is the fundamental unit of executable functionality.
An action is the specification of an executable statement and is the fundamental unit of processing or behavior in an activity node that represents some transformation in the modeled system.
An action forms an abstraction of a computational procedure which is an atomic execution and therefore completes without interruption.
In contrast, an activity is a more complex collection of behavior that may run for a long duration.
An action is a result of a system state change and is realized by sending a message to an object or modifying a link or a value of an attribute.