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 help 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., Angelus.

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
Big Bad is a term originally used by the Buffy the Vampire Slayer television series to describe a major recurring adversary, usually the chief villain or antagonist in a particular broadcast season. It has since been used to describe annual villains in other television series, and has also been used in scholarly work discussing Buffy the Vampire Slayer.

This pattern in action:

Movie/The Origin

 * Big Bad
 * Lothos
 * Little Bad
 * Benny Jacks  (initially)
 * Amilyn

Season 1

 * Big Bad
 * The Master
 * Little Bad
 * Darla (initially)
 * Order of Aurelius

Season 2

 * Big Bad
 * Angelus
 * Spike (initially)
 * Little Bad
 * Drusilla

Season 3

 * Big Bad
 * Mayor Richard Wilkins
 * Little Bad
 * Faith Lehane
 * Mr. Trick (initially)

Season 4

 * Big Bad
 * Adam
 * Dr. Maggie Walsh (initially)
 * Little Bad
 * The Initiative

Season 5

 * Big Bad
 * Glory
 * Little Bad
 * The Knights of Byzantium

Season 6

 * Big Bad
 * Dark Willow
 * Warren Mears (initially)
 * The Trio (initially)

Season 7

 * Big Bad
 * The First Evil
 * Little Bad
 * Caleb

Season 8

 * Big Bad
 * Twilight
 * Little Bad
 * Twilight Group
 * The Master/The Seed Of Wonder

Season 9

 * Big Bad
 * Simone Doffler
 * Severin

Season 10

 * Big Bad
 * D'Hoffryn
 * Little Bad
 * The Soul Glutton
 * The Sculptor
 * The Mistress

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

 * Big Bad
 * Wolfram & Hart

Season 2

 * Big Bad
 * Darla (initially)
 * Wolfram & Hart
 * Little Bad
 * Drusilla

Season 3

 * Big Bad
 * Daniel Holtz
 * Little Bad
 * Sahjhan

Season 4

 * Big Bad
 * Jasmine
 * The Beast (initially)
 * Little Bad
 * Angelus

Season 5

 * Big Bad
 * Circle of the Black Thorn
 * Little Bad
 * Lindsey McDonald (initially)
 * Marcus Hamilton

After the Fall

 * Big Bad
 * Demon Lords (initially)
 * Vamp Gunn
 * Little Bad
 * Senior Partners

IDW series

 * Big Bad
 * Myresto Mor
 * Little Bad
 * Rowant Mor

Angel and Faith

 * Big Bad
 * Whistler
 * Little Bad
 * Nash
 * 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 & Hart

Behind the scenes

 * At the end of the Season 7 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".
 * Darla was the only member of the Whirlwind to not play a significant antagonist role in the second season of Buffy the Vampire Slayer.

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.