Kaijū(怪獣,kaijū?) is a Japanese word that literally translates to "monster," and is used to refer to a genre of tokusatsu (special effects-based) 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 Kanojijo.
In the Japanese language original of Cardcaptor Sakura, Sakura's brother Toya likes to tease her by regularly calling her "kaiju", relating to her noisily coming down from her room for breakfast every morning.
In the second season of Star Wars: The Clone Wars, there is a story arc composed of two episodes entitled "The Zillo Beast" and "The Zillo Beast Strikes Back", mostly influenced by Godzilla films, in which a huge reptilian beast is transported from his homeworld Malastare to the city-covered planet Coruscant, where it breaks loose and goes on a rampage.
In Star Wars Episode VI: Return of the Jedi, the rancor was originally to be played by an actor in a suit similar to the way how kaiju films like Godzilla were made. However, the rancor was eventually portrayed by a puppet filmed in high speed.
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. Guillermo del Toro directed the Treehouse of Horror XXIV couch gag which made multiple references to Godzilla and other Kaiju-based characters, including his own Pacific Rim characters.
In the 2009 film Crank: High Voltage, there is a sequence parodying Kaiju films using the same practical effects techniques used for Tokusatsu films such as miniatures and suitmation.
In the 2013 film Pacific Rim, "Kaiju" is the moniker bestowed upon giant inter-dimensional monsters that invade Earth and attempt to exterminate humanity.