bug

The word bug has a number of senses in English. In American English is usually used to describe very small animals (such as insects, spiders, snails, slugs, etc). (In British English the words 'creepy crawlies' are often used.) From this meaning stem many others.

Zoology

Bug is often used in vernacular to refer generally to any small, terrestrial arthropod, sometimes taken to include creatures like snails and slugs as well. Since such insects are often irritating and frustrating (mosquitoes, flies and cockroaches being prime examples of such), this word has also come to refer to something which is irritating or frustrating. For instance, a person might say someone (or a problem) is "bugging" (irritating or frustrating) them.
Bug is a term used in Australia as a name for two species of edible crustaceans of the family Scyllaridae, the Moreton Bay bug (Thenus orientalis) and the Balmain bug (Ibacus peronii).
Bug is also a precise scientific term that refers to insects of the order Hemiptera. The term true bugs is often used to remove ambiguity.
Since it refers to small animals, the term bug is also occasionally (and inaccurately) used to refer to microscopic life forms and viruses. Somebody might refer to having caught the "pneumonia bug", for instance.

Technology

A further extension of the sense of "small lifeforms" is to small surveillance devices. This can be seen as an extension of the existing meaning of something which is small and irritating (the possible presence of surveillance bugs acts as a detriment to free speech in any context). Alternatively, a bugging device could also be likened to a "fly on the wall" - another kind of bug. See bugging.
The term bug is often used in a computing context. It describes unwanted behavior of a computer program or electronic device. More specific terms are failure or defect. See computer bug (including origin of the usage) or debugging.
In a flight instrument, a bug is a manually positioned marker or pointer which is set to remind the pilot where the needle on the instrument should be pointing. This avoids the need to hold settings in memory, allowing the pilot to simply keep the needle aligned with the bug. The term may have arisen because of the superficial similarity to the appearance of an insect.

Gambling

In gambling jargon, a bug is a small holdout device that can secretly be attached at the underside of the card table for the purpose of cheating. A card cheat will use a bug to conceal extra cards under the table for further use.
In some variants of poker, the bug is a restricted wild card, which can either represent an ace or fill a gap in a straight or flush.

Other

To bug someone is a synonym for annoying someone.
Bug is a name for a special kind of Morse code key originally manufactured by the Vibroplex company.
Bug is a common nickname for the VW Beetle.
Bug is the name of two rivers in eastern Europe, the Western and Southern Bug
BUG is computer magazine published in Croatia.
A bug is a TV station or TV network logo used as a watermark, usually in the lower right of the video.
A Bug is a mixed-breed dog. It is the result of the breeding of a Pug and a Boston Terrier.
Bugs was a British television drama series which ran for four seasons from April 1995 to August 1999.
Bugs Bunny is a well-known cartoon character.