Beta

From Sonic Retro

In SEGA's naming terminology, a beta refers to game where all major game modes and features are closed; game is now being heavily tested for bugs by Sega of America's QA department. Betas are one of a few different types of prototypes.

The much more known Nick Arcade and Simon Wai Sonic 2 prototypes are not betas, as commonly believed—they are indeed alpha ROMs, with Nick Arcade possibly being a pre-alpha.