The use of live nonhuman animals for scientific research and experimentation. Animal testing has been a cornerstone of biomedical research, having contributed greatly to the understanding of human biology, to the unraveling of the complex underlying causes and effects of disease, and to the development and testing of effective and safe means of diagnosis and treatment. However, the use of sentient beings as disease models and test subjects (Fig. 1) raises a significant ethical dilemma between animal harm and human benefits. Faced with this dilemma, the mainstream ethical, social, and political stance has been to deem it acceptable only when no alternatives are available, if aimed at answering meaningful scientific or medical questions, and provided animal harm is minimized. Some individuals, however, advocate that no animal use in science is morally justified, regardless of any potential benefits. Further fueling the discussion, the medical value of animal experiments has been increasingly challenged. Scientists, companies, regulators, and funders are responding by improving transparency regarding why and how animals are used in science, while working continuously to raise standards and make animal experiments more reliable, humane, and relevant. With this as a background, the 3Rs principle for replacement, reduction, and refinement offers a meaningful and ever-improving approach to ease the pressing ethical and scientific challenges at hand. See also: Bioethics; Biology; Biotechnology; Experiment; Medicine; Pharmaceuticals testing; Scientific methods

A discipline concerned with the application of ethics to biological problems, especially in the field of medicine. Bioethics is a broad term that encompasses the environmental, ethical, legal, and social implications of agriculture, life sciences, and medicine. The rapidity of progress in genetics and genome mapping in recent decades has revolutionzied many aspects of the life sciences, as well as provided new tools for use in technology, and has in the process also raised many questions for society. Bioethical issues arising from modern advances in science include genetic engineering of microorganisms, plants, and animals; in vitro fertilization; cloning; xenotransplantation; gene therapy; genetic testing; and stem cell research (Fig. 1). See also: Agricultural science (plant); Biotechnology; Cloning; Genetic engineering; Genetic mapping; Genetics; Human genetics; Human genome; Medicine; Reproductive technology; Stem cells; Transplantation biology; Xenotransplantation

The empirical method is not sharply defined. It is generally characterized by the collection of a large amount of data before much speculation as to their significance, or without much idea of what to expect, and is to be contrasted with more theoretical methods in which the collection of empirical data is guided largely by preliminary theoretical exploration of what to expect. The empirical method is necessary in entering hitherto completely unexplored fields, and becomes less purely empirical as the acquired mastery of the field increases. Successful use of an exclusively empirical method demands a higher degree of intuitive ability in the practitioner.

An active and intentional act or practice of putting to death, in a relatively painless way, of persons suffering severely from medical conditions that are incurable, untreatable, or irreversible. Euthanasia is an act by a person, usually a physician or other health care provider, actively and intentionally to end another person's life by some medical means, such as a lethal injection, at that person's explicit request (see illustration). Notably, the definition of euthanasia excludes passive decisions, such as withholding or withdrawing life-sustaining or life-prolonging treatments, which have often been termed cases of passive euthanasia; however, this is a misnomer. Similarly, nonvoluntary euthanasia and involuntary euthanasia, which are often-used terms for the act of administering lethal drugs without the patient's explicit request, constitute misnomers. By definition, euthanasia is active and voluntary. In addition, it is an act or practice that raises bioethical and legal issues. See also: Bioethics; Critical care medicine; Death; Disease; Medicine

The test of a hypothesis under controlled conditions. The experiment is one of the distinctive tools of the scientist. It enables the scientist to put questions to nature and receive answers. These answers lead to new problems whose solutions require more complex experiments (seeking smaller differences), improved techniques, detailed plans, and better analysis of results.

A tentative supposition with regard to an unknown state of affairs, the truth of which is thereupon subject to investigation by any available method, either by logical deduction of consequences which may be checked against what is known, or by direct experimental investigation or discovery of facts not hitherto known and suggested by the hypothesis. The formulation of a hypothesis about a solution to a problem or answer to a question is part of the scientific method. Testing the hypothesis—typically by running experiments, gathering observations, and analyzing results via statistical methods—is done to ultimately determine whether the hypothesis is true or false. The data must be objectively evaluated to determine whether they support the hypothesis. If the data do not support the hypothesis, then the investigator must reject the hypothesis. See also: Experiment; Philosophy of science; Scientific methods

The accumulated body of scientific and technical writing published to serve the informational needs of, primarily, scientists, engineers, and research workers, and, to the extent that it can be understood, the general public. This vast literature is worldwide in origin, international in language, diverse in subject content, varied in form, uneven in quality, and expensive (Fig. 1). An ever-increasing portion of it is being produced, stored, and retrieved or published electronically, and is being made available through the Web as well as in print-on-paper format.

A reference sample used in comparing lengths, areas, volumes, masses, and the like. The measures employed in scientific work are based on the international units of length, mass, and time—the meter, the kilogram, and the second—but decimal multiples and submultiples are commonly employed. Prior to the development of the international metric system, many special-purpose systems of measures had evolved and many still survive, especially in Great Britain and the United States, where metric units have not come into common use. Multiples and submultiples in these systems are usually nondecimal; some are based on successive doubling or halving; others involve subdivision of 60 or 12; still others have multiples and submultiples of unknown origin. A few common systems are listed here; others can be found in engineering hand-books.

A system of measurement units used in scientific work and for everyday applications by most countries, with the notable exception of the United States. A prime advantage of the metric system is its international acceptance as a standard of measurement, providing a common measurement language for most of the world's population.

The body of knowledge that concerns the fundamental nature, function, development, and use of formal models in science and technology. In its most general sense, a model is a proxy. A model is one entity used to represent some other entity for some well-defined purpose. Examples of models include: (1) An idea (mental model), such as the internalized model of a person's relationships with the environment, used to guide behavior. (2) A picture or drawing (iconic model), such as a map used to record or present geological data (Fig. 1), or a solids model used to design a machine component. (3) A verbal or written description (linguistic model), such as the protocol for a biological experiment or the transcript of a medical operation, used to guide and improve procedures. (4) A physical object (scale model, analog model, or prototype), such as a model airfoil used in the wind-tunnel testing of a new aircraft design, or an electronic circuit used to simulate the neural activity of the brain. (5) A system of equations and logical expressions (mathematical model or computer simulation), such as the mass- and energy-balance equations that predict the end products of a chemical reaction, or a computer program that simulates the flight of a space vehicle. Models are developed and used to help hypothesize, define, explore, understand, simulate, predict, design, or communicate some aspect of the original entity for which the model is a substitute. See also: Airfoil; Chemistry; Hypothesis; Wind tunnel