Big Bad

A Big Bad is a term applied (mostly by fans and the show's creators, but also spoken in the Buffyverse) to describe major villains on Buffy the Vampire Slayer and Angel. Specifically, it is usually accepted to refer to the single villain that, in each season of Buffy, is the dominant and final one.

The Big Bad is typically not only dangerous (most of the Big Bads threaten some version of apocalypse) but also helps articulate the major themes of the season. Some of the Big Bads are built up and present from the season's beginning (or earlier) - i.e., The Master - and others are sprung as a surprise near the end of the season - i.e., Angel.

For some of the show, the seasons followed a pattern in which lesser villains (presumed to be the Big Bad) are introduced, only to later lead to the more powerful surprise Big Bad: Near the beginning of the season, a villain was introduced who appeared to be a Big Bad and absorbs much of the heroes' attention. Then, suddenly, this "Little Bad" is rid of (typically killed) and replaced by the Big Bad, usually about halfway through the season.

Buffy Little Bads/Big Bads
In Buffy, the little bad/big bad pattern is rarely perfect, and the specific instances are often disputed. Unlike the other "Little Bads", Spike does not die (although it is well known Joss Whedon intended the character to be offed, until James Marsters' portrayal convinced the show's creators to keep him around); the Mayor's presence is well known before Mr. Trick is killed; and Warren dies much later in the season than the other "Little Bads". However, Whedon has stated that the true Big Bad of season six was "life."

Nevertheless, creative arguments can be made for different interpretations of the Little Bad/Big Bad pattern. For example, Faith may be interpreted as the true replacement for Mr. Trick in Season 3.

This pattern in action:

Movie/The Origin: Benny Jacks->Amilyn/+Lothos Season 1: Order of Aurelius/The Master Season 2: -Anointed One->+Spike->-Drusilla/+Angelus Season 3: Mr. Trick->-Faith/+The Mayor

Season 4: -The Initiative/+Maggie Walsh->+Adam Season 5: +Glory

Season 6: Nerd Trio->Warren Mears-&gt;Dark Willow Season 7: -Caleb/ +The First Season 8: -Twilight Group -> The Master/ The Seed -> +Twilight

Season 9: +Simone Doffler / -Severin

Angel Little Bads/Big Bads
For Angel, the law firm Wolfram & Hart was the Big Bad present in the series; however, there were other major recurring threats that arose, those of which were either associates of Wolfram & Hart or a third party with its own agenda. While the format of the series didn't stick to an established Big Bad the way Buffy did, several characters and elements have represented significant season arcs, using the "Big Bad" term frequently. The pattern of this was as follows:

Season 1: +Wolfram & Hart/ -Lindsey McDonald

Season 2: +Darla/ -Drusilla -> +Wolfram & Hart/

Season 3: -Sahjhan/ +Daniel Holtz

Season 4: -The Beast -> -Angelus -> +Jasmine

Season 5: Lindsey McDonald/Marcus Hamilton->+Circle of the Black Thorn

After the Fall: Demon Lords->Vamp Gunn/Senior Partners

IDW series: Jamaerah/Rowant Mor

Angel and Faith: Whistler/ Nash and Pearl

Spike Little Bads/ Big Bads
Spike received a lot of comic book series that have disputable canon, therefore creating official new Big Bads and treating it as some kind of a second spin-off.

Spike: Asylum - Mosaic Wellness Center

Spike: Shadow Puppets - Smile Time (show)

Spike: After the Fall - Non

Spike: The Devil You Know - Tansy Fry

Spike (IDW series) - Drusilla/John/Wolfram and Hart

Behind the scenes

 * At the end of the season seven episode "Lessons," the First Evil appears in the form of the previous six seasons' Big Bads, in reverse chronological order: Warren Mears (season 6), Glory (season 5), Adam (season 4), Mayor Wilkins (season 3), Drusilla (season 2), and The Master (season 1).
 * The term was originally coined by Buffy in the episode "Bewitched, Bothered and Bewildered", in which she describes Angelus as "the big bad thing in the dark". The phrase may originate in various fairy tales (particularly "3 Little Pigs" and the related song) about the "big bad wolf". It has also been expanded as "Big Bad Evil Guy" (abbreviated as BBEG). It is first used on screen in the simpler "Big Bad" form in Season 3, in the episode "Gingerbread" where Buffy says that an occult symbol is harmless, "not a big bad". Spike has also nicknamed himself "Big Bad".

In other media
The term big bad has been adopted by several other series.
 * Wikipedia lists several examples.
 * The Supernatural Wiki has a page on Big Bads, listing out their respective seasonal antagonists.
 * The TV Tropes Wiki lists many more examples.