A villain is a "evil" character in a story, whether a historical narrative or, especially, a work of fiction. The villain is usually the bad guy or antagonist, the character who fights against the hero or protagonist. A female villain is sometimes called a villainess. Random House Unabridged Dictionary defines villain as "a cruelly malicious person who is involved in or devoted to wickedness or crime; scoundrel; or a character in a play, novel, or the like, who constitutes an important evil agency in the plot."
What makes a villain a villain many times comes down to personal opinion on what defines "good" and "evil." Yet, there are a few acts that are universally accepted as villainous: betrayal, murder and deception as well as abuse. Genocide, child abuse, violence toward women and children, mass murder, torture, and crimes against humanity are almost always seen as "evil" acts.
Furthermore, many villains do not see themselves as "evil." They only appear as such because they are in opposition to the 'good' guys. Their "evilness" may come from the ways in which they achieve their goals, not the goals themselves. It should be noted that the stereotypical cackling maniac intent on destruction for the sake of destruction is a character-type that while still popular in fiction is beginning to fade away in favor of villains that display the same varied depths of emotions as heroes do: making them more three-dimensional characters than they were originally.
Evolution Of Villainy
In the early days of media portrayal of villains (during the 18th, 19th, and 20th centuries) it was common for a villain to be portrayed as the archetypal "cackling madman" with little to no redeeming qualities. However, as audiences and society in general, matured villains became a less superficial in nature and the idea of villainy was evolved. It became popular for a villain to share similar qualities to a hero. This gave birth to the anti-hero and anti-villain as well as the tragic villain. In fact in modern times many writers are encouraged to steer away from outright villainous individuals and instead are inclined to create antagonists that are more three-dimensional so as to serve a wider audience. It is worth noting many villains in cartoons and children's media however retain the stereotypical villain. This is due to children's shows and cartoons being geared toward younger individuals who are unable to fully understand deeper, more philosophical concepts associated with anti-heroes and tragic villains.