What Is A Kaiju Film?
A film that is made in the West (Japan and other Asian countries) that is about (a) kaiju(s). Not all kaiju films are made in the west, for example, Gorgo, Pacific Rim, and The Host are kaiju films, but they were not made in Asia.
However, not all giant monster movies are kaiju films, at least to die-hard kaiju fans. Films like Cloverfield, the 1998 American Godzilla film, and Tarantula are not true kaiju films to die-hard fans, but are instead just normal giant monster movies. This is because their titular creatures are not called kaiju within the film or any spin-offs, because they are not made in the west, and they do not use the same style. However, to the Japanese, any film with any type of monster in it is a Kaiju Movie, so in a technical standpoint, any movie with a monster is a kaiju film.