act

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

• noun The process of doing or performing something.
• noun Something done or performed; a deed.
• noun Law Something done that has legal significance.
• noun A statute or other law formally adopted by a legislative body.
• noun A formal written record of proceedings or transactions.
• noun One of the major divisions of a play, opera, or film.
• noun A performance or entertainment usually forming part of a longer presentation.
• noun The actor or actors presenting such a performance.
• noun A manifestation of intentional or unintentional insincerity; a pose.
• intransitive verb To play the part of; assume the dramatic role of.
• intransitive verb To perform (a role) on the stage.
• intransitive verb To behave like or pose as; impersonate.
• intransitive verb To behave in a manner suitable for.
• intransitive verb To behave or comport oneself.
• intransitive verb To perform in a dramatic role or roles.
• intransitive verb To be suitable for theatrical performance.
• intransitive verb To behave affectedly or unnaturally; pretend.
• intransitive verb To appear or seem to be.
• intransitive verb To carry out an action.
• intransitive verb To operate or function in a specific way.
• intransitive verb To serve or function as a substitute for another.
• intransitive verb To produce an effect.
• idiom (be in on the act) To be included in an activity.
• idiom (clean up (one's) act) To improve one's behavior or performance.
• idiom (get into the act) To insert oneself into an ongoing activity, project, or situation.
• idiom (get (one's) act together) To get organized.

from The Century Dictionary.

• To do, perform, or transact.
• To represent by action; perform on or as on the stage; play, or play the part of; hence, feign or counterfeit: as, to act Macbeth; to act the lover, or the part of a lover.
• To perform the office of; assume the character of: as, to act the hero.
• To put in action; actuate.
• To do something; exert energy or force in any way: used of anything capable of movement, either original or communicated, or of producing effects. Specifically
• To put forth effort or energy; exercise movement or agency; be employed or operative: as, to act vigorously or languidly; he is acting against his own interest; his mind acts sluggishly.
• To exert influence or produce effects: perform a function or functions; operate: as, praise acts as a stimulant; mind acts upon mind; the medicine failed to act; the brake refused to act, or to act upon the wheels.
• To be employed or operate in a particular way; perform specific duties or functions: as, a deputy acts for or in place of his principal; he refused to act on or as a member of the committee.
• To perform as an actor; represent a character; hence, to feign or assume a part: as, he acts well; he is only acting.
• He is a man of sentiment, and acts up to the sentiments he professes. Sheridan, School for Scandal, i. 2. Synonyms Act, Work, Operate. These words agree in expressing the successful exertion of power. In their intransitive use they are sometimes interchangeable: as, a medicine acts, works, or operates; a plan works or operates. Where they differ, act may more often refer to a single action or to the simpler forms of action: as, a machine works well when all its parts act. Act may also be the most general, applying to persons or things, the others applying generally to things. Operate, may express the more elaborate forms of action. Work may express the more powerful kinds of action: as, it worked upon his mind.
• noun An exertion of energy or force, physical or mental; anything that is done or performed; a doing or deed; an operation or performance.
• noun A state of real existence, as opposed to a possibility, power, or being in germ merely; actuality; actualization; entelechy.
• noun The soul, according to the Aristotelians, is the act, that is, is the entelechy or perfect development of the body. So God is said to be pure act, for Aristotle says, “There must be a principle whose essence it is to be actual (η%148ς ἠ ον)σία ἐνε)ργεια),” and this is by many writers understood to mean “whose essence is to be active.” In the phrase in act, therefore, act, though properly meaning actuality, is often used to mean activity.
• noun A part or division of a play performed consecutively or without a fall of the curtain, in which a definite and coherent portion of the plot is represented: generally subdivided into smaller portions, called scenes.
• noun The result of public deliberation, or the decision of a prince, legislative body, council, court of justice, or magistrate; a decree, edict, law, statute, judgment, resolve, or award: as, an act of Parliament or of Congress; also, in plural, proceedings; the formal record of legislative resolves or of the doings of individuals.