action

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The state or process of acting or doing.
• noun Something that is done or accomplished; a deed.
• noun Organized activity to accomplish an objective.
• noun The causation of change by the exertion of power or a natural process.
• noun Habitual or vigorous activity; energy.
• noun Behavior or conduct.
• noun Law A proceeding brought before a court to obtain relief; a lawsuit.
• noun Armed encounter; combat.
• noun An engagement between troops or ships.
• noun The most important or exciting work or activity in a specific field or area.
• noun A movement or a series of movements, as of an actor.
• noun Manner of movement.
• noun The appearance of animation of a figure in painting or sculpture.
• noun The series of events and episodes that form the plot of a story or play.
• noun A series or number of fast-moving, exciting, or dangerous events, especially in a movie.
• noun The operating parts of a mechanism.
• noun The manner in which such parts operate.
• noun The manner in which a musical instrument can be played; playability.

from The Century Dictionary.

• To bring a legal action against.
• noun In mech., the sum of the average momenta of the elements of a moving system, each multiplied by the distance through which it moves.
• noun In dynamo-electric machines, wasteful internal circuits in the pole-pieces or cores; eddy, parasitic, or Foucault currents.
• noun The process or state of acting or of being active, as opposed to rest; change of which the cause lies within the subject; activity; active exertion; energy manifested in outward acts, as contrasted with contemplation, speculation, speaking, or writing: as, a man of action.
• noun An event considered as predicated of its cause; an act, usually in a complex or an inclusive sense; that which is done about or in relation to anything; a specific performance, proceeding, or course of conduct: as, a good or a bad action; actions speak louder than words; the action of a deliberative body.
• noun An exertion of power or force; the real relation of a cause to its effect; causality; influence; agency; operation; impulse: as, the action of wind upon a ship's sails.
• noun Manner of moving; kind of motion or physical performance: as, this horse has fine action; the action of a machine.
• noun In rhetoric, gesture or gesticulation; the deportment of the speaker, or the accommodation of his attitude, voice, gestures, and countenance to the subject, or to the thoughts and feelings expressed.
• noun In poetry and the drama, the connected series of events on which the interest of the piece depends; the main subject or story, as distinguished from an incidental action or episode. Unity of action is one of the dramatic unities.
• noun In physiology: Any one of the active processes going on in an organized body; some manifestation of vital activity; the performance of a function: as, the action of the stomach or the gastric juice on the food; a morbid action of the liver.
• noun A more or less complex muscular effort.
• noun In law: A proceeding instituted in court by one or more parties against another or others to enforce a right, or punish or redress a wrong: distinguished from judicial proceedings which are not controversial in form, as the probate of a will.
• noun Such a proceeding under the forms of the common law, as distinguished from a chancery suit and a criminal prosecution.
• noun The right of bringing an action: as, the law gives an action for every claim.
• noun In the fine arts: The appearance of animation, movement, or passion given to figures by their attitude, position, or expression, either singly or concurrently.
• noun The event or episode represented or illustrated by a work of art.
• noun A military fight; a minor engagement between armed bodies of men, whether on land or water: of less importance than a battle. See battle.
• noun In machinery: The mechanism of a breech-loading gun by which it is opened to receive the charge.
• noun That part of the mechanism of a pianoforte, an organ, or other similar instrument by which the action of the fingers upon the keys is transmitted to the strings, reeds, etc.
• noun A share in the capital stock of a company; in the plural, stocks, or shares of stock.
• noun In firearms, when the locks are bedded into the stock alone. E. H. Knight.

from the GNU version of the Collaborative International Dictionary of English.

• noun 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 on one body by another; agency; activity; operation
• noun An act; a thing done; a deed; an enterprise. (pl.): Habitual deeds; hence, conduct; behavior; demeanor.
• noun The event or connected series of events, either real or imaginary, forming the subject of a play, poem, or other composition; the unfolding of the drama of events.
• noun Movement.
• noun (Mech.) Effective motion; also, mechanism.