Kaiju (怪獣) is a Japanese word that literally translates to "strange creature". However, the word kaiju has been translated and popularly defined in English as "monster".
Specifically, it is used to refer to characters in the genre of tokusatsu entertainment. Kaiju films and other media usually showcase giant monsters or robots of any form attacking a major metropolitan city (usually Japanese) or engaging another (or multiple) monsters in battle.
The most (in)famous kaiju is Godzilla. Other notable kaiju include King Kong, Gamera, Mothra, King Ghidorah, Mechagodzilla and Rodan... among many, many others.
This is a family of games whose theme revolves around this specific type of monster genre.
Daikaiju (大怪獣) or "giant kaiju", specifically meaning the larger variety of monsters, is a more precise moniker describing what is commonly referred to as kaiju.