Definition of Action

Definition Last Updated 17-Dec-2015 12:51

An action is a unit of activity for which no internal description is provided, i.e. it is an event (potentially of extended duration), characterised entirely by a specified change in state. Defined thus, in terms of changes of state, an action may be seen as the occurrence of an exercise of a capability; actions may be performed by a performer – but they might also simply be reactions, i.e. an effect arising from a particular cause.

It may be possible, and indeed it usually is, to divide an action into smaller actions; whilst this may appear to convert an action into an activity (given that the description of a thing should be fixed unless it is acted upon), activity is the fundamental concept: an activity may be described as an action only for the actor who is unable to interrupt it.


Action is a defined term of Enterprise Architecture. Action is a defined term of Business Analysis.


Article Last Updated 17-Dec-2015 12:51

The characterisation of action applies equally to what is done intentionally, such as picking up a pen, and what just happens, such as two raindrops coalescing on the window on a rainy November day in England.

As far as picking up my pen is concerned, the action is reaching out and grasping (considered as a whole); in the case of the raindrops, the action is touching, which then causes them to coalesce.

Whilst it might be thought that an action should have a single, primary actor who does something that causes something else to happen, it is easy to construct counterexamples.

Consider two atoms of hydrogen (H) of constant velocities that collide to form a molecule (H2). If either were stationary, it might be natural to call the other the primary actor since it does the “colliding”. However, (inertial) motion being relative, there is no reason to prefer either as the actor, so there is none in this case.

Contrariwise, when there is a genuine asymmetry there may be case for identifying some actor as the primary actor; in general, however, an action need not have a primary actor.

An action is minimally specified as a transformation by its initial and final states; whilst every transformation is an effect of something, and therefore has a cause, the primary actor may not be identifiable.

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