International Encyclopedia of Systems and Cybernetics

1. "A point in space-time, precisely localized by its three spatial coordinates and its time coordinate" (JP. LUMINET, 1987).

2. "A change in one or more structured properties of either an object, a system, an environment or a relationship between them over a time period of specified duration" (R.L. ACKOFF & F.E. EMERY, 1972, p.25).

LUMINET comments to his most general definition that "the separation between two events remains absolute, whatever reference system is used ".

In accordance with ACKOFF and EMERY definition, an event must frequently be an unforeseen happening.

From a very basic viewpoint, G.L. FARRE expresses that "events are the traces left by these transformations"(i.e. of energy)

As energy acts at different levels in nature, we have gravitational, chemical, electro-mechanical, biological, or microphysical and quanta events. (1998b, p. 262)

An event does not always fit into the sequence of situations which seemed possible within the formerly forecasted or awaited ones. As stated by J.van GIGCH, unique events, far away from standard statistical deviations, can be enormously important (1978, p.212).

In the study of a complex system, it obliges us to question our knowledge, or at least the model that we had formerly built.(in an explicit or implicit way).

Such failures in forecasting are generally results of:

- the ignorance or disregard of some basic aspect of the system or its interactions with its environment;

- the ignorance or misappreciation of the irreductibility of long term transformations to medium or short term ones.

3. For the French biologist J.P. CHANGEUX an event is "Any modification in the environment or of the system itself that may modify its dynamic stability (Homeostasis)" (1972, p.37).

CHANGEUX's definition refers only to living systems. It seems however that it can be applied to social systems and, possibly in the future to artificial distributed intelligent systems, if it becomes possible to endow them with the ability to modify their collective behavior by learning through behavior rules, without any rigidly imposed algorithmic control. (see "Artificial Life").

Following this definition, the event may oblige the system to create a new response (i.e. to learn) and not merely to cross-over from one state to any other, previously visited or known.

From a constructivist and physio-psychological viewpoint, in R. FISCHER words: "… environmental events are specified as internal states that are - in mammalian brains – the neural system 's own symbols, as these states stand in regular relation to events in the world and signify potentials for action" (1992, p.219).

An event can be destructive if the system does not discover some efficient response to it. It may also lead merely to a broadening of the system 's states repertory. In this case, the system maintains its organizational closure.

As a third possibility, the system could be so radically modified by the event that its basic identity becomes altered. This could lead to emergence (if the event is a massive increase of energy supply), or to evolution, if the system's basic organization is altered by a nonlethal mutation.