A neg is a penalty given to a player who has interrupted the question with an incorrect answer. Negs are used in all forms of collegiate quizbowl, including NAQT and ACF, and in most forms of high school quizbowl. Some exceptions at the high school level include the PACE NSC format, Illinois Scholastic Bowl, and the OAC format. The verb "to neg" (word forms: negged, negging, etc.) also refers to the act of answering a tossup incorrectly, regardless of whether doing so incurs a penalty or not. The term "interrupt" by itself can refer to the act of negging. Although this term is seldom used in colloquial speech, some official language still retains use of the phrase, most notably in NAQT stats keeping.
In all known formats which deduct 5 points for negs, they only apply when the wrong answer is given by the first team to buzz on the question and the question has not yet been completed; the moderator then continues reading the rest of the question for the other team. If the question is over, or the other team is buzzing after the first team has already negged, or both, then there is no penalty for a wrong tossup answer.
If you are playing a game of quizbowl and the other team has negged a tossup, it is usually strategically wise to wait until the moderator finishes reading the entirety of the rest of the tossup before buzzing in, so as to maximize one's chance of giving the correct answer. Failure to do so is called vulturing.
It's Academic and Panasonic use a different system, in which the entire value of the tossup is lost by answering incorrectly at any time, and the other teams are not allowed to buzz on that question.