The term Kaiju Eiga means literally 'Monster Movie' and refers to Japanese monster and science-fiction fantasy films. (loose interpretations allow for non-kaiju films in the category, such as War in Space, etc.). 'Kaiju' is the Japanese word for strange or unusual creature and is usually applied to Japanese monsters only. (monsters from otrher cultures usually don't fit the description). As such, a kaiju is a very specific type of monster, more fantastical than realistic, perhaps. The term 'Dai Kaiju' means giant kaiju, the original of which is Gojira, of course!

The Millennium alien spaceship lands on a skyscraper (from Gojira 2000)