Meanings
Biology
Informally, most arthropods, except marine crustaceans, including individuals or species of insect
spiders
scorpion
mite
tick
centipede
millipede
woodlouse
Medical
Bacterium or any microorganism that causes illness and has a superficial resemblance to an insect, or bug, when viewed through a microscope
Technology
Software bug, or a software defect
Errors in hardware with close affinity to software
Covert listening device, or bug, commonly used in espionage and in police investigations
Web bug, a technique to track who has read a web page or e-mail
Acronyms
BUG Backpackers' Ultimate Guide (travel guide series)
BUG BCL (Binary Control Language) Users Group
BUG Back Up Gun
BUG BSD User Group (Berkeley software distribution OS family UNIX-based)
BUG Back Up Generator (Boeing 777)
BUG Bicycle User Groups
BUG Bringing Up Grades (school program)
BUG Banner Users Group
BUG Backlight, Uplight and Glare (outdoor lighting)
BUG Brooklyn Union Gas Company
BUG Bottom-Up Greedy (pre-assignment technique)
BUG BeOS Users Group
BUG Benthic Unattended Generator (Microbial Fuel Cell)
BUG Big Ugly Goof (programming mistake)
BUG Boulder Colorado .NET User Group (Boulder, CO)
BUG Beer Users Group (various locations)