The cause of all bad happenings in a story. A Big Bad could be a character with Evil Plans or it could be an omnipresent situation, such as a comet heading towards the Earth. In a serial story, the Big Bad exerts an effect across a number of episodes, even an entire season.
This trope is not a catch-all term for the biggest, ugliest villain of any given story. In fact, it doesn't have to be a villain at all, as we just said. If it is a villain, though, it should be identified correctly; the badass leader of the outlaw gang that causes the most personal trouble is not the Big Bad. The railroad tycoon who is using the gang as muscle is the Big Bad. The Man Behind the Man is very common for this trope, leaving the reveal of the big bad as The Chessmaster behind it all and proving themselves far more clever and resourceful than the Villain of the Week. Sometimes the Big Bad is the grand enemy of an entire franchise. At other times, the Big Bad is an Arc Villain who causes trouble for a period of time only to be replaced by another Big Bad.
In its most general form, a Big Bad will be at the center of the Myth Arc rather than just any Story Arc, but this doesn't always have to be the case; when you look at a season-long story or a major Story Arc and you can identify one problem being the cause of everything, that is the Big Bad.
The term "Big Bad" was popularized in Buffy the Vampire Slayer. It was characteristic of Buffy's Big Bads for their identity or nature, or even the fact that they are the Big Bad at all, to remain unclear for a considerable time. Occasionally, characters would even refer to themselves as "the Big Bad" — whether they were a true Big Bad or just a Big Bad Wannabe is another matter. The structure of Buffy placed the Big Bad as being crucial to the Half-Arc Season, half the episodes are filler dealing with unrelated enemies while the other half involved the ongoing Myth Arc with the Big Bad. Each season can easily be defined by who the Big Bad was.
If a show has a series of Big Bad jeopardies, they can function like a series of Monsters of the Week that take more than one week to finish off. If there is a Legion of Doom, you can expect the Big Bad to be involved somehow. They're probably sorted by power, with the strongest for last, following the Sorting Algorithm of Evil.
In tabletop gaming circles, the Big Bad is often referred to as the BBEG (Big Bad Evil Guy).
Evil Overlord, Diabolical Mastermind, The Chessmaster, Arch-Enemy, The Man Behind the Man, and often Manipulative Bastard are specific types of villains who are liable to show up as Big Bads. If they're a Magnificent Bastard or Hero Killer, the good guys are in big trouble. The heroic counterpart of this character is the Big Good, who will very often be the focus of this character's attention over The Hero at the beginning of a series. If a work of fiction is conspicuously lacking a Big Bad, it may be a case of No Antagonist.
See also Big Bad Duumvirate for two (or more) Big Bads working together. Sometimes a Big Bad will get their start as a servant to another villain — if that's the case, they're a Dragon Ascendant. If the character who fills the role of Big Bad in most meaningful ways is nominally subordinate to someone else (someone significantly less menacing by comparison), they are a Dragon-in-Chief. If the story has many Big Bads at once who don't work together, see Big Bad Ensemble. The Big Bad Shuffle occurs when there are multiple candidates for the Big Bad position. If the Big Bad doesn't start out as bad but develops over the course of the story, it's Big Bad Slippage. If the Big Bad of one section of a work doesn't die on being defeated and stays around as a character in a different plot role (reformed or not), that's Ex-Big Bad.
The Big Bad of a story is not always the most powerful or oldest existing evil force. Perhaps an evil presence along the lines of an Eldritch Abomination overshadows the work's setting, but is mainly divorced from the story's events — that would be the Greater-Scope Villain.
It is one of the most well-known tropes on the TV Tropes community, being the first to have over sixty thousand wicks, and is currently the most wicked trope on the site. This is probably because it's incredibly common.
