There are a couple of different ways to say demon in the Japanese language. Akuma is the word used to describe Western style demons, and oni describes Japanese style demons. Deemon is an English loanword that is commonly used in anime.
|Demon (English loanword)|
Interesting facts about Japanese demons
Japanese mythology and Shintoism is rich in demons, ghosts, oni and other supernatural beings. If you are interested then check out this list of creatures from Japan.