/bəg/
n. An unwanted and unintended property of a program or hardware, esp. one that causes it to malfunction.
Antonym of feature. Examples:
"There's a bug in the editor: it writes things out backwards." "The system crashed because of a hardware bug." "Fred is a winner, but he has a few bugs" (i.e., Fred is a good guy, but he has a few personality problems).
Historical note: Some have said this term came from telephone company usage, in which "bugs in a telephone cable" were blamed for noisy lines, but this appears to be an incorrect folk etymology. Admiral Grace Hopper (an early computing pioneer better known for inventing COBOL) liked to tell a story in which a technician solved a persistent glitch in the Harvard Mark II machine by pulling an actual insect out from between the contacts of one of its relays, and she subsequently promulgated bug in its hackish sense as a joke about the incident (though, as she was careful to admit, she was not there when it happened). For many years the logbook associated with the incident and the actual bug in question (a moth) sat in a display case at the Naval Surface Warfare Center. The entire story, with a picture of the logbook and the moth taped into it, is recorded in the 'Annals of the History of Computing', Vol. 3, No. 3 (July 1981), pp. 285--286.
The text of the log entry (from September 9, 1945), reads "1545 Relay #70 Panel F (moth) in relay. First actual case of bug being found". This wording seems to establish that the term was already in use at the time in its current specific sense. Indeed, the use of 'bug' to mean an industrial defect was already established in Thomas Edison's time, and 'bug' in the sense of an disruptive event goes back to Shakespeare! In the first edition of Samuel Johnson's dictionary one meaning of 'bug' is "A frightful object; a walking spectre"; this is traced to 'bugbear', a Welsh term for a variety of mythological monster which (to complete the circle) has recently been reintroduced into the popular lexicon through fantasy role-playing games.
In any case, in jargon the word almost never refers to insects. Here is a plausible conversation that never actually happened:
"There is a bug in this ant farm!"
"What do you mean? I don't see any ants in it."
"That's the bug."
[There has been a widespread myth that the original bug was moved to the Smithsonian, and an earlier version of this entry so asserted. A correspondent who thought to check discovered that the bug was not there. While investigating this, your editor discovered that the NSWC still had the bug, but had unsuccessfully tried to get the Smithsonian to accept it -- and that the present curator of the History of American Technology Museum didn't know this and agreed that it would make a worthwhile exhibit. Thus, the process of investigating the original-computer-bug bug may have fixed it in an entirely unexpected way, by making the myth true! -- ESR]