An interrupt penalty or neg is a penalty (almost universally 5 points) assessed to a player who has interrupted a tossup with an incorrect answer. Negs are used in all extant forms of collegiate quizbowl, including NAQT and ACF and in many forms of high school quizbowl. The verb "to neg" (word forms: negged, negging, etc.) can either mean incurring an interrupt penalty or any incorrect tossup answer regardless of whether it incurs a penalty.
In all known team-based formats that issue interrupt penalties, a penalty can only be incurred if the answer is (a) incorrect, (b) the first incorrect answer given, and (c) given before the end of the question has been read. As a result, there can only be one penalty per question. Wrong answers after the end, and second wrong answers, do not incur penalties. (At the IPNCT, during group rounds, penalties are given for all incorrect answers before the end of the question.)
If a team has negged, it is usually strategic for players on the other team to wait until the end of the tossup to attempt buzzing so that they get more information. The major exceptions are when time is running low and the latter team is slightly behind, or when powers are available and the game is so close that the (usually) 5-point difference between getting power and not getting power could affect the outcome of the game. Answering a tossup after a neg but before the tossup is over is called vulching (short for vulturing), and doing so for power is called power vulching.
Some formats (notably It's Academic and Panasonic) assess a penalty for any incorrect answer. The penalty is often the full value of the question, and it often prevents other teams from attempting to answer the question. This is not usually called a neg, at least not without qualification.
(source: College Bowl Valhalla Facebook page, authored by Tom Michael?)
The history of negs traces back to the College Quiz Bowl radio show.
Originally, players were allowed to interrupt toss-up questions. The frequency of interrupted questions surprised the producers and host Allen Ludden. After the first few shows, the rules were changed to require teams to wait for the toss-up to be read before signalling; with correct answers given before the question was finished not counted. By January 16, 1954, interruptions were reintroduced and the neg was in effect.
The first neg was awarded to Brown, on the January 9, 1954 episode of College Quiz Bowl. Video of the Jan. 2, 1954 match between Brown and Michigan, before the initial rule change.