The definitions used in this glossary of terminology either have been provided by the authors of the articles, or have been extracted wholly or in part, or paraphrased from the following sources: The American Medical Association Encyclopedia of Medicine, Charles B. Clayman, MD, Medical Editor, Random House, New York, 1989; Biotechnology from A to Z, 2d Edition, William Bains, Oxford University Press, New York, New York, 2002; A Dictionary of Genetics, 6th Edition, Robert C. King and William D. Stansfield, Oxford University Press, New York, New York, 2002; Dorland's Illustrated Medical Dictionary, 29th and 30th Editions, W. B. Saunders Company, Philadelphia, 2000, 2003; Genes VII, Benjamin Lewin, Oxford University Press, New York, New York, 2000; The Gale Encyclopedia of Genetic Disorders, Volumes I and II, Stacey L. Blachford, Ed., Thomson Learning, New York, New York, 2002; The Merriam-Webster Dictionary, Merriam-Webster, Inc., Springfield, Massachusetts, 1997; Molecular Biology of the Cell, 3rd Edition, Bruce Alberts, et al., Garland Publishing, 1994; The Random House Dictionary of the English Language, Unabridged Edition, 1966; Webster's Ninth New Collegiate Dictionary, 1991.


1. A constant in a mathematical expression that distinguishes specific cases, having a definite, fixed value in one case but different values in other cases. For example, in the equation of a straight line, y = mx +b, m and b are parameters that specify a particular straight line; changing m changes the slope of the line, while changing b changes the point at which the line crosses the y-axis.

2. In statistics, a value that specifies one of the various members of a family of probability distributions, e.g., the mean or variance of a normal distribution. A parameter is often thought of as the "true value" or "population value" as opposed to the observed value or sample value.

3. A variable whose measure is indicative of a quantity or function that cannot itself be precisely determined by direct methods; e.g., blood pressure and pulse rate are parameters of cardiovascular function, and the level of glucose in blood and urine is a parameter of carbohydrate metabolism.

4. A characteristic element; factor.