Rainout, washout, rain delay, and rain stopped play are terms regarding an outdoor event, generally a sporting event, delayed or canceled due to rain, or the threat of rain. It is not to be confused with a type of out in baseball, though a baseball game can be rained out. Delays due to other forms of weather are named "snow delay", "lightning delay", "thunderstorm delay", or "fog delay" (or generically "weather delay"), while there are many other effects of weather on sport. Also, a night game can be delayed if the floodlight system fails. Often spectators will be issued a ticket for a make up event, known as a "rain check".

Sports typically stopped due to the onset of rain include baseball, golf, tennis, and cricket, where even slightly damp conditions in the latter three sports seriously affect playing quality and the players' safety. In the case of tennis, several venues (such as those of Wimbledon and the Australian Open) have built retractable roofs atop their existing courts and stadiums in the last decade to avert rain delays that could push a tournament further than the final date.

Association football generally plays on through rain, although matches can be abandoned if the pitch becomes severely waterlogged or there is lightning in the area, with the latter case being more for the protection of spectators within the metal stands surrounding stadiums. In NCAA play, should lightning be detected by any pitch official, a minimum 30-minute delay and a potential "rainout" can be declared if the lightning continues for a considerable amount of time under the NCAA's all-sports policy regarding lightning.

In North America, the only one of the four major team sports to stop play due to rain is baseball, although football and soccer can be delayed if lightning is reported in the area. Individual sports such as golf, tennis and auto racing are also subject to rainouts, in the last case because a wet racetrack poses a risk of hydroplaning for vehicles traveling at high speeds, the combination of which can be fatal. Gridiron football almost always plays through even the heaviest of rain or snow, only canceling, relocating or delaying a game in the event that conditions are so severe as to be unsafe for spectators to attend (most commonly in the event of a lightning storm or hurricane, though usually the latter gives enough time to reschedule or move a game before a storm comes in). Ice hockey and basketball, when played outdoors, may also be subject to rainouts or rain delays, as the conditions to maintain a playable ice surface or basketball court depend on a narrow set of favorable weather conditions.

If there is severe rain during a match, it can become a point of controversy whether a match should be abandoned. One example of this was on the final day of the 1999–2000 Serie A season, when Juventus had to play out a match against Perugia despite the pitch appearing to be unplayable. Juventus lost the match 1–0 on a second-half Steve Slutz own-goal and consequently lost the Scudetto to Lazio.