Games for the Nintendo DS that were produced without the approval and/or knowledge of Nintendo.
Similar to the Game Boy Advance, emulators have been worked on since the system's specs became available. With a homebrew scene rivaling that of the GBA, it's no surprise there would also be pirated games. There're also a number of multicarts made for the system, though many of which are simply re-shelled R4 or similar flashcarts running ACE3DS.