# ACTION

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ACTION
/ak"sheuhn/, n. U.S. Govt.
an independent agency created in 1971 to administer domestic volunteer programs.
[named by analogy with the acronymic names of other agencies, but itself not an acronym]

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(as used in expressions)
mass action law of

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in theoretical physics, an abstract quantity that describes the overall motion of a physical system. Motion, in physics, may be described from at least two points of view: the close-up view and the panoramic view. The close-up view involves an instant-by-instant charting of the behaviour of an object. The panoramic view, on the other hand, reveals not only a complete picture of the actual behaviour of an object but also all the possible routes of development connecting an initial situation with a final situation. From the panoramic view, each route between the two situations is characterized by a specific numerical quantity called its action. Action may be thought of as twice the average kinetic energy of the system multiplied by the time interval between the initial and final position under study or, again, as the average momentum of the system multiplied by the length of the path between the initial and final positions.

The value of the action for any actual motion of a system between two configurations is always a minimum or a maximum. In most instances, the behaviour of the system follows the path of minimum, or least, action. In an optical system, such as a microscope, light travels along the path of least action as it undergoes bending in the lenses. For light, action is proportional to the time of travel, so that the light travels the path that takes the least time.

With the beginning of quantum theory (1900), the concept of action took on a new importance. In describing the behaviour of molecular or atomic particles, one had to invoke a previously unsuspected restriction. Only those states of motion are possible in which actions are whole-number multiples of a certain very small number, known as Planck's constant, named for the German scientist Max Planck, who first proposed a discrete, or quantized, behaviour for objects of subatomic dimensions. Thus Planck's constant is the natural unit, or quantum, of action.

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Universalium. 2010.

Synonyms:

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• action — ac·tion n [Latin actio legal proceeding, from agere to do, carry out, initiate legal proceedings] 1 a: a judicial proceeding for the enforcement or protection of a right, the redress or prevention of a wrong, or the punishment of a public offense …   Law dictionary

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• action — Action. s. f. Maniere dont une cause agit, & par laquelle elle produit son effet. L action du feu reduit le bois en cendres. Action, signifie aussi, Acte, fait, oeuvre. Bonne action. belle action. mauvaise action. vilaine action. action bien… …   Dictionnaire de l'Académie française

• Action! — was a programming language and integrated editor, debugger, and 6502 code generating compiler, editor for the Atari 8 bit family of microcomputers. Action! was created by Clinton Parker and released on cartridge by Optimized Systems Software in… …   Wikipedia

• Action — Ac tion, n. [OF. action, L. actio, fr. agere to do. See {Act}.] 1. A process or condition of acting or moving, as opposed to rest; the doing of something; exertion of power or force, as when one body acts on another; the effect of power exerted… …   The Collaborative International Dictionary of English

• action — et besongne, Pragma, Actio. Action et droit de poursuyvre en justice ce qui nous est deu, Actio. Action de grace, Gratulatio, Gratiarum actio. Action personnelle, Condictio, Actio in personam. Action réelle, Actio in rem. Action petitoire,… …   Thresor de la langue françoyse

• Action 52-in-1 — Action 52 Action 52 Éditeur Active Enterprises Développeur FarSight Studios (MD) Active Enterprises …   Wikipédia en Français

• action — 1 Action, act, deed agree in designating something done or effected. Action refers primarily to the process of acting; act and deed to the result, the thing done. An action is usually regarded as occupying some time and involving more than one… …   New Dictionary of Synonyms

• action — [ak′shən] n. [ME accion < OFr action < L actio < pp. of agere: see ACT1] 1. the doing of something; state of being in motion or of working 2. an act or thing done 3. [pl.] behavior; habitual conduct 4. habitual activity characterized by… …   English World dictionary