Mario & Luigi is a series of RPG games starring the titular duo of brothers, created by Nintendo for handheld consoles, and it's a sub-series of both the Mario series and the Luigi series, therefore technically being a crossover between the two.
The released games are:
Mario & Luigi RPG (November 17, 2003), Game Boy Advance. Called Mario & Luigi: Superstar Saga outside Japan.
Mario & Luigi RPG 2x2 (November 28, 2005), Game Boy Advance. Called Mario & Luigi: Partners in Time outside Japan. Re-released for WiiU Virtual Console in 2015.
Mario & Luigi RPG 3!!! (February 11, 2009) known as Mario & Luigi: Bowser's Inside Story outside Japan.
Mario & Luigi RPG 4: Dream Adventure (July 12, 2013), Nintendo 3DS. Known as Mario & Luigi: Dream Team outside Japan.
Mario & Luigi: Paper Jam (December 3, 2015), Nintendo 3DS. A crossover with the Paper Mario series.