This tag should be used for questions that involve giant monsters, which are often created through atomic incidents. The name can be translated as "strange beast" and is a Japanese film genre.
The word "kaiju" describes a genre of japanese films that involve giant monsters that were created through atomic incidents. The most well known of these films is "gojira", which was published in 1954 and translated as "Godzilla". The genre started in a time when American movies involving giant monsters, such as "King Kong", were popular in Japan and because of the second World War people in Japan feared atomic weapons more than ever before.
The word can be translated as "strange beast" and originally referred to monsters in Japanese folklore, though there are no depcitions of such kaiju in folklore.
Kaiju can sometimes be used to refer to giant monsters that are modelled after animals, mythological creatures, plants or a mix of all of the aforementioned. This is a big difference to the monsters like "King Kong" from the western film studios, which are simply natural animals scaled up. They can either play a role as the protagonist or antagonist, or they are simply an uncontrollable force of nature.