Game Development Dictionary
A ren'ai game, also known as a dating sim, is a popular genre of game in Japan but hasn't made it to western gaming very much. In this game the player plays one main character, (usually) male, and the game objective is to court and impress one or more (usually) female NPCs. Gameplay usually relies heavily on dialogue choices and may contain sim or adventure elements. There is not usually any combat. Perhaps the clearest western examples of this genre would be the Leisure Suit Larry games. There are also X-rated versions of these games known as hentai games, h games, or ecchi games.