Kaiju is a Japanese word that literally translates to "Strange Creature". However, the word has been translated to and defined in English as "Monster".
It is used specifically to refer to a genre of tokusatsu entertainment; kaiju films usually showcase monsters attacking major establishments or engaging other monsters in battle. The most famous is Godzilla; others include Gamera, King Ghidorah, Mechagodzilla and Rodan. The term Ultra-kaiju is longhand in the Ultra Series. Related terms include monster movies and films featuring giant and/or single monsters referring to roughly humanoid ones and Giant kaiju meaning larger ones. Kaiju is also used as the name for the titanic bioweapons in Pacific Rim.
Kaiju are typically modeled after conventional and/or mythological creatures; however, there are exotic examples. Chōjin Sentai Jetman featured monsters based on traffic lights, faucets and tomatoes; Kamen Rider Super-1 included monsters based on household objects such as umbrellas and utility ladders.
The term is used in English to describe monsters from tokusatsu and 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 Frankenstein vs. Baragon.
Kaiju are sometimes depicted as cannon fodder serving greater evils or warriors to greater villains. Others have neutral alignments only seeking to destroy buildings and other structures. During early tokusatsu heroic monsters were rarely seen in daikaiju eiga films and it was not until later when tokusatsu productions introduced kaiju that aided heroes, saved innocents and/or demonstrated complex personalities. These kaiju adopted many classic monster traits appearing as "Misunderstood Creatures". Some kaiju hung out with heroes and provided comedy relief in contrast to the darker approach from 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 to be depicted in multiple roles and having those around him react in different ways depending on how the creatures themselves were presented.