A Big Bad is a term coined by fans of Buffy the Vampire Slayer as a name for a particularly-strong enemy Buffy Summers fights in the last few episodes of each season. Some fans can confuse it with an enemy that Buffy was plagued with throughout the season, for example in Season Two the season's main enemies were Spike and Drusilla, but the Big Bad was Angelus who Buffy was troubled with in the last few episodes, significantly more than with Spike and Drusilla. In Season Two's case, Spike and Drusilla would be considered the season's Little Bad, a term also coined by fans of the show.
SEASON Big Bad Little Bad
ONE The Master Luke and Darla
TWO Angelus Spike and Drusilla
THREE The Mayor Faith
FOUR Adam The Initiative
FIVE Glory Knights of Byzantium
SIX Willow The Troika
SEVEN Caleb The First Evil