A programming language, originally designed for Dartmouth's experimental timesharing system in the early 1960s, which for many years was the leading cause of brain damage in proto-hackers. Edsger W. Dijkstra observed in Selected Writings on Computing: A Personal Perspective that “It is practically impossible to teach good programming style to students that have had prior exposure to BASIC: as potential programmers they are mentally mutilated beyond hope of regeneration.” This is another case (like Pascal) of the cascading lossage that happens when a language deliberately designed as an educational toy gets taken too seriously. A novice can write short BASIC programs (on the order of 10-20 lines) very easily; writing anything longer (a) is very painful, and (b) encourages bad habits that will make it harder to use more powerful languages well. This wouldn't be so bad if historical accidents hadn't made BASIC so common on low-end micros in the 1980s. As it is, it probably ruined tens of thousands of potential wizards.
[1995: Some languages called “BASIC” aren't quite this nasty any more, having acquired Pascal- and C-like procedures and control structures and shed their line numbers. —ESR]
BASIC stands for “Beginner's All-purpose Symbolic Instruction Code”. Earlier versions of this entry claiming this was a later backronym were incorrect.