Kaiju (怪獣, kaijū?) is a Japanese word that literally translates to "strange beast". However, the word Kaiju has been universally translated and defined into English as "monster" or "giant monster" and refers to Science fiction films from Japan featuring unnatural creatures of immense size. Kaiju films usually showcase Kaijus or giant monsters of any form attacking a major Japanese city or squaring off with one another.
The most famous Kaiju is Godzilla. Other well known Kaiju include Mothra, Gamera, Rodan, and King Ghidorah. The term ultra-kaiju is short-hand for monsters in the Ultra Series.
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 monster), specifically meaning the larger variety of monsters.
Read more about Kaiju: Concept, Examples of Kaiju, Kaiju Film Creators, Major Kaiju Films