Kaiju(怪獣,kaijū?) is a Japanese word that literally translates to "strange creature," and is used to refer to a genre of tokusatsu entertainment. Kaiju films usually showcase monsters of any form, usually attacking a major Japanese city or engaging another (or multiple) monster(s) in battle.
Related terms include kaijū eiga(怪獣映画,kaijū eiga?, monster movie), a film featuring giant monsters or a single monster; kaijin(怪人?, referring to roughly humanoid monsters); and daikaiju(大怪獣,daikaijū?, giant kaiju), specifically meaning the larger variety of monsters.
Kaiju are sometimes depicted as cannon fodder serving a greater evil. Some kaiju are elite warriors which serve as the right-hand man to the greater villain and are destroyed by the heroic forces. Others have a neutral alignment, only seeking to destroy buildings and other structures. During the early eras of tokusatsu, "heroic" monsters were rarely seen in daikaiju eiga films, and it was not until later when television tokusatsu productions began using kaiju which aided the hero, saved civilians, or demonstrated some kind of complex personality. These kaiju adopted many classic monster traits, appearing as the "Misunderstood Creature". Some kaiju hung out with the heroes and provided comedy relief, in contrast to the darker approach to these characters from more mature franchises, like Kamen Rider. Godzilla, arguably the most well known of the daikaiju, has played the roles of hero, villain, and force of nature in the course of his existence, one of the few kaiju of any type to be depicted in multiple roles and having those around him react in different ways, depending on how the creature itself was being presented in the films.
In the second season Star Wars: The Clone Wars, there are two episodes entitled "The Zillo Beast" and "The Zillo Beast Strikes Back", mostly influenced by Godzilla films, in which a huge reptilian/insectoid beast is transported to the city-covered planet Coruscant, where it breaks loose and goes on the rampage.
The Simpsons: Treehouse of Horror VI - Attack of the 50-Foot Eyesores, Homer goes to Lard Lad Donuts unable to get a "Colossal Doughnut" as advertised, he steals Lard Lad's Donut awaking other giant advertising statues and coming to life to terrorize Springfield. When Lard Lad awakes he makes a Godzilla roar.
In the 2013 film Pacific Rim, "Kaiju" is the moniker bestowed upon giant, extradimensional monsters that invade Earth and attempt to exterminate humanity.