A mini-series is a short series of comic books with a definite plot, storyline, starting and ending point. Unlike a regular series, which may last for years (depending on the sales or popularity of the character or group), a mini-series ranges from two issues to more than fifty issues. The average number of issues in a mini-series ranges from three to six issues. A mini-series is also known as a limited series or a maxi-series (12 or more issues).
One could argue that the first type or concept of a mini-series were the Silver Age titles The Brave and the Bold and Showcase. Both series introduced new characters or groups in a series of issues. If the characters or groups became popular enough then they would receive a regular ongoing series of their own. The mini-series concept began in 1979 with the DC Comics three issue mini-series entitled World of Krypton. The purpose of the World of Krypton was to tell a story of life on Krypton prior to its destruction. Other Superman theme-related mini-series followed such as Krypton Chronicles, The Phantom Zone, and Superman: The Secret Years.
Other theme-related series followed concerning established characters such as Batman with the mini-series The Untold Legend of the Batman, While others introduced new characters such as Amethyst, Princess of Gemworld and Jemm, Son of Saturn. The most famous and ground breaking mini-series of the 1980s were Marvel Comic's Secret Wars and DC Comics' Watchmen.
The mini-series may credited with the creation of the graphic novel concept with the four issue mini-series by Frank Miller, Batman: The Dark Knight Returns. Other mature related mini-series followed with the Green Arrow: The Longbow Hunters and Blackhawk.