Kaiju is a Japanese word that literally translates to "strange creature". However, the word kaiju has been translated and defined in English as "monster".
Specifically, it is used to refer to a genre of tokusatsu entertainment. Kaiju films usually showcase monsters of any form attacking a major Japanese city or engaging another (or multiple) monsters in battle.
Related terms include monster movie, a film featuring giant monsters or a single monster, referring to roughly humanoid monsters and giant kaiju, specifically meaning the larger variety of monsters.
The term Kaiju is also used to describe the giant monsters that appeared in the 2013 film, Pacific Rim. The Kaiju are creatures engineered by aliens and used as bioweapons to destroy humanity.
Kaiju are typically modeled after conventional animals, insects or mythological creatures; however, there are more exotic examples. Chōjin Sentai Jetman features monsters based on traffic lights, faucets and tomatoes; Kamen Rider Super-1 includes a whole army of monsters based on household objects such as umbrellas and utility ladders.
While the term kaiju is used in English to describe monsters from tokusatsu and Japanese folklore, monsters such as vampires, werewolves, Frankenstein's monster, mummies and zombies would fall into this category. In fact, Frankenstein's monster was once a kaiju in the film Frankenstein vs. Baragon, which was created by Toho.
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.