A third party developer is a company that makes video games and then publishes them for any hardware platform it would like to, provided the parent company of said platform will allow it (i.e. Squaresoft may want to make Gameboy Advance titles, but Nintendo won't let them unless they make GameCube titles as well.) There are usually several different development teams within the company, each working on seperate products.

Examples of this would be Capcom, Konami and Electronic Arts.

For information on first party developers and second party developers, click the links.