a generally insane person (or animal in cartoons) whose sole goal usually involves one of the following:
a. world domination
b. world conquest
c. world demolition
d. mass death and destruction
he or she usually seeks this in order to achieve fame or fortune. the mastermind is usually defeated seconds before one of the above goals is achieved in comics by a superhero or in movies by an average joe incidentally involved due usually to a love triangle between the mastermind, him and his love interest or a land dispute where the mastermind decides the perfect place for his conquest is the land owned by the average joe.
The Joker, the Brain, squirrels, Lex Luthor, etc. are all evil masterminds.