From The Art and Popular Culture Encyclopedia
An antagonist (from Greek ἀνταγωνιστής - antagonistēs , "opponent, competitor, enemy, rival") is a character, group of characters, or institution, that represents the opposition against which the protagonist must contend. In other words, 'A person, or a group of people who oppose the main character, or the main characters.' In the classic style of stories wherein the action consists of a hero fighting a villain/enemy, the two can be regarded as protagonist and antagonist, respectively.
The antagonist may also represent a major threat or obstacle to the main character by their very existence, without necessarily deliberately targeting him or her.
Examples in both film and theatre include Sauron, the main antagonist in The Lord of the Rings, who constantly battles the series' protagonists, and Tybalt, an antagonist in Romeo and Juliet, who slays Mercutio and whose later death results in the exiling of the play's protagonist, Romeo. However, despite the antagonist often being classified the "bad guy", the antagonist can be a good person (one notable example being Macduff from Macbeth)