A soap opera, or soap for short, is a typically long-running radio or television serial, frequently characterized by melodrama, ensemble casts, and sentimentality. The term "soap opera" originated from radio dramas originally being sponsored by soap manufacturers. The term was preceded by "horse opera", a derogatory term for low-budget Westerns.

BBC Radio's The Archers, first broadcast in 1950, is the world's longest-running radio soap opera. The longest-running current television soap is Coronation Street, which was first broadcast on ITV in 1960, with the record for the longest running soap opera in history being held by Guiding Light, which began on radio in 1937, transitioned to television in 1952, and ended in 2009.

One of the defining features that makes a television program a soap opera, according to Albert Moran, is "that form of television that works with a continuous open narrative. Each episode ends with a promise that the storyline is to be continued in another episode". In 2012, Los Angeles Times columnist Robert Lloyd wrote of daily dramas:

Although melodramatically eventful, soap operas such as this also have a luxury of space that makes them seem more naturalistic; indeed, the economics of the form demand long scenes, and conversations that a 22-episodes-per-season weekly series might dispense with in half a dozen lines of dialogue may be drawn out, as here, for pages. You spend more time even with the minor characters; the apparent villains grow less apparently villainous.

Soap opera storylines run concurrently, intersect and lead into further developments. An individual episode of a soap opera will generally switch between several concurrent narrative threads that may at times interconnect and affect one another or may run entirely independent to each other. Episodes may feature some of the show's current storylines, but not always all of them. Especially in daytime serials and those that are broadcast each weekday, there is some rotation of both storyline and actors so any given storyline or actor will appear in some but usually not all of a week's worth of episodes. Soap operas rarely conclude all the current storylines at the same time. When one storyline ends, there are several other story threads at differing stages of development. Soap opera episodes typically end on some sort of cliffhanger, and the season finale (if a soap incorporates a break between seasons) ends in the same way, only to be resolved when the show returns for the start of a new yearly broadcast.

Evening soap operas and those that air at a rate of one episode per week are more likely to feature the entire cast in each episode, and to represent all current storylines in each episode. Evening soap operas and serials that run for only part of the year tend to bring things to a dramatic end-of-season cliffhanger.

In 1976, Time magazine described American daytime television as "TV's richest market", noting the loyalty of the soap opera fan base and the expansion of several half-hour series into hour-long broadcasts in order to maximize ad revenues. The article explained that at that time, many prime time series lost money, while daytime serials earned profits several times more than their production costs. The issue's cover notably featured its first daytime soap stars, Bill Hayes and Susan Seaforth Hayes of Days of Our Lives, a married couple whose onscreen and real-life romance was widely covered by both the soap opera magazines and the mainstream press at large.