doujin game; fangame; hobbyist video game
Refers to video games created by hobbyists or small circles, often self-published at events like Comiket. Can be original works or derivative fangames.
He releases a new doujin game every year at Comiket.
This doujin game is a derivative work based on a famous anime.
Compound of 同人 (doujin, 'same person/group; coterie') and ゲーム (game). The term 同人 originally referred to a group of like-minded people, and by extension, self-published works by such groups.