In American football, an unfair act is a foul that can be called when a player or team commits a flagrant and obviously illegal act that has a major impact on the game.
All of the major American football codes include some form of unfair act rule. In all cases, the definition is deliberately vague, giving the officials great latitude in defining such an act and enforcing penalties for such acts; officials are allowed to award any penalty, up to and including forfeiture of the game. The National Federation of State High School Associations, however, also includes the general rule that all acts are legal unless otherwise explicitly stated; thus, the unfair act rule is only invoked in cases when specific rules have clearly been broken, but the penalty for the foul itself would still be less than the result of the play had the unfair act not occurred.
The National Football League defines two types of unfair acts, a palpably unfair act and an extraordinarily unfair act. The latter is for acts so extraordinary that the NFL Commissioner can levy fines, draft forfeiture, or suspension. The Commissioner could also reverse the game results, but this has never occurred.
The 1954 Cotton Bowl Classic featured a notorious use of the rule. Rice University's Dicky Moegle broke free for an apparent touchdown run, but Alabama's Tommy Lewis entered the field and tackled Moegle. This would be illegal participation, for which the penalty then was 5 yards. However, the officials declared a palpably unfair act and credited the touchdown anyway. Modern college and NFL officials are explicitly permitted to award a touchdown under such circumstances.
The NFL's rule on deliberate fouls is open-ended but covers only "successive or repeated fouls to prevent a score." It would only be a palpably unfair act for the defense to commit deliberate fouls, preferring the certainty of a small penalty over the uncertainty of a score attempt, if the defense did so again after an official's warning. On November 6, 2016, near the end of the first half, the San Francisco 49ers deliberately held pass receivers, forcing the New Orleans Saints to settle for a short field goal. The NFL instructed its officials that this would be a palpably unfair act subject to a 15-yard penalty if repeated. On November 27, 2016, the Baltimore Ravens took a safety, conceding 2 points of their 5-point lead. They committed numerous holding penalties to ensure that they could exhaust the final 11 seconds of the game. This was not a palpably unfair act because it did not recur (and was not done "to prevent a score" but in fact while conceding points). Beginning in the 2017 NFL season, deliberately committing fouls to manipulate the game clock will constitute unsportsmanlike conduct.