Kaijū (怪獣, kaijū) (from Japanese "strange beast") is a Japanese film genre that features monsters, usually attacking a major cities and engaging the military and other monsters in battle. It is a subgenre of tokusatsu (special effects-based) entertainment.
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.
Godzilla is an example of a daikaiju; others include Mothra, King Ghidorah, Mechagodzilla, Rodan, Gamera, Gyaos, Daimajin and Gappa. The term ultra-kaiju is longhand for kaiju in the Ultra Series.
Toho has produced a variety of kaiju films over the years (many of which featured Godzilla and Mothra) but other Japanese studios contributed to expanding the genre in Japan by producing films and shows of their own, including Daiei Film Co., Ltd., Kadokawa Pictures, Tsuburaya Productions, Shochiku and Nikkatsu studios.