Brand of graphics processing units from AMD, included with other manufacturers' graphics cards. Ask questions about configuring or troubleshooting in Ubuntu.
The Radeon brand was started by ATI, which was then bought out by AMD. Radeon competes directly with the GeForce brand owned by Nvidia.
On Ubuntu, Radeon drivers and support have historically been questionable as ATI would release drivers for Linux about half as often as for Windows. However in recent years this trend has changed and Linux driver releases are typically in line with Windows.