Osu! is a free-to-play rhythm game primarily developed, published, and created by Dean "peppy" Herbert. Inspired by iNiS' rhythm game Osu! Tatakae! Ouendan, it was written in C# on the .NET Framework, and was released for Microsoft Windows on 16 September 2007. The game has throughout the years been ported to macOS, Linux, Android and iOS.

Rhythm game or rhythm action is a genre of music-themed action video game that challenges a player's sense of rhythm. Games in the genre typically focus on dance or the simulated performance of musical instruments, and require players to press buttons in a sequence dictated on the screen. Many rhythm games include multiplayer modes in which players compete for the highest score or cooperate as a simulated musical ensemble. Rhythm games often feature novel game controllers shaped like musical instruments such as guitars and drums to match notes while playing songs. Certain dance-based games require the player to physically dance on a mat, with pressure-sensitive pads acting as the input device.