In the United States, federal and most state courts allow class action suits. Such suits have figured prominently in civil-rights litigation and in other cases brought to further social and economic reform. In recent decades they have been employed notably by groups of consumers and others seeking to affix liability for harm caused by various products, especially through manufacturers' negligence. Major litigation against the producers of the Dalkon shield (an intrauterine device; see birth control), of Agent Orange (a herbicide used as a defoliant in the Vietnam War), and of asbestos insulation has involved class action suits.

Since the 1980s such suits have been under attack, along with negligence litigation in general, with opponents, mainly conservatives and business interests, arguing that many lawsuits are frivolous and that awards are out of proportion to the offense in some juridictions. A study published in 2004 that reviewed several hundred state and federal class action lawsuits from 1993 to 2002 found that, adjusted for inflation, the average annual award in such suits varied but did not progressively increase, while the median award was relatively constant. At the same time, however, federal court data showed that the number of class action lawsuits doubled from 1997 to 2002.

In U.S. politics, an organization whose purpose is to raise and distribute campaign funds to candidates seeking political office. PACs rose to prominence after the Federal Election Campaign Act (1971) limited the amount of money any corporation, union, or private individual could give to a candidate. PACs were able to circumvent these limits by soliciting smaller contributions from a much larger number of individuals. During the late 20th and early 21st centuries the vast amounts of money raised by PACs greatly increased the cost of running for office and led to efforts to reform this method of financing campaigns.

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Fundamental law of chemical kinetics (the study of rates of chemical reactions), formulated in 1864–79 by the Norwegian scientists Cato M. Guldberg (1836–1902) and Peter Waage (1833–1900). The law states that the reaction rate of any simple chemical reaction is proportional to the product of the molar concentrations of the reacting substances, each raised to the power corresponding to the number of molecules of that substance in the reaction.

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Type of breech mechanism that was key to developing an effective repeating rifle. It combines the firing pin, a spring, and an extractor, all housed in or attached to the bolt. A projecting handle with a round knob moves the bolt back and forth. As the bolt is thrust forward, it pushes a cartridge into the chamber and cocks the piece. The trigger releases the spring-driven firing pin inside the bolt. After firing, the extractor on the head of the bolt removes the spent cartridge and ejects it. The bolt then moves a new cartridge from the magazine.

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In the U.S., the effort to improve the employment and educational opportunities of women and members of minority groups through preferential treatment in job hiring, college admissions, the awarding of government contracts, and the allocation of other social benefits. First undertaken at the federal level following passage of the landmark Civil Rights Act of 1964, affirmative action was designed to counteract the lingering effects of generations of past discrimination. The main criteria for inclusion in affirmative action programs are race, sex, ethnic origin, religion, disability, and age. The Supreme Court of the United States placed important limitations on affirmative action programs in its 1978 ruling in Regents of the University of California v. Bakke; several subsequent Supreme Court decisions (e.g., Adarand Constructors v. Pena in 1995 and Texas v. Hopwood in 1996) imposed further restrictions. In 1996 California voters passed Proposition 209, which prohibited government agencies and institutions from discriminating against or giving preferential treatment to individuals or groups on the basis of race, sex, colour, ethnicity, or national origin. Similar measures were subsequently passed in other states. In 2003, in two landmark rulings involving admission to the University of Michigan and its law school, the U.S. Supreme Court reaffirmed the constitutionality of affirmative action but ruled that race could not be the preeminent factor in such decisions.

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Sports events characterized by high speed or high risk. Such sports include aggressive inline skating, wakeboarding, street luge, skateboarding, and freestyle bicycle events (wherein tricks such as back flips are performed on a bicycle). Many of the events—snowboarding, skateboarding, and freestyle biking are examples—are performed on ramps and inclines or in a halfpipe, some with walls as high as 50 feet, which allows the athletes to get air, or achieve the height necessary to, for example, rotate on a bicycle. Many of these sports were made popular by the X Games (championship competitions sponsored by the cable-television network ESPN). The 1998 and 2002 Olympic Games featured extreme snowboarding events in a halfpipe, in which boarders performed jumps, rotations, and mid-air maneuvers.

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Brief (about one-thousandth of a second) reversal of electric polarization of the membrane of a nerve or muscle cell. Stimulation of the cell by certain chemicals or by sensory receptor cells causes depolarization of the membrane, permitting an impulse to move along the nerve fibre (in nerve cells) or causing the cell to contract (in muscle cells).

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