An Action element describes a basic process or transformation that occurs within a system. It is the basic functional unit within an Activity diagram. Actions can be thought of as children of Activities. Both represent processes, but Activities can contain multiple steps or decomposable processes, each of which can be embodied in an Action. An Action cannot be further broken down or decomposed. 

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OMG UML Specification

The OMG UML specification (UML Superstructure Specification, v2.0, p. 230) states:

"An action is a named element that is the fundamental unit of executable functionality. The execution of an action represents some transformation or processing in the modeled system, be it a computer system or otherwise."

The OMG UML specification (UML Superstructure Specification, v2.0, p. 301) also states:

"An action may have sets of incoming and outgoing activity edges that specify control flow and data flow from and to other nodes. An action will not begin execution until all of its input conditions are satisfied. The completion of the execution of an action may enable the execution of a set of successor nodes and actions that take their inputs from the outputs of the action."