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Coding error in a computer program that prevents it from functioning as designed. Most software companies have a quality-assurance department which is charged with finding program bugs while the program is in development (debugging); bugs are also often detected by means of beta testing (testing of a product, often by potential consumers, before it is placed on the market). The term originated in a computer context in 1945 when a moth flew into and jammed an electrical relay of the Harvard Mark II computer; it was extracted and taped into the log book with the inscription First actual case of bug being found (the term having previously been used for other kinds of mechanical defects).
This entry comes from Encyclopædia Britannica Concise. For the full entry on bug, visit Britannica.com.