What is a "Bug"?

An unwanted and unintended property of a program or piece of 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).

read more bug history, how the word evolved to be used in CompLang.