The Beta phase usually comes before the release of a game and is generally defined as the game being ‘feature-complete’. The game does everything it’s going to do, though it may not look right just yet. This means art, design, and engineering are only fixing bugs, glitches, crashes, or any other problems that arise from the implementation of various features. Anyone could play a game in beta and get a full feel for the game as a whole.
The Beta phase is also when a company brings in external testers to receive their feedback and criticism. That means that for many of these players it is their first impressions and for that reason beta can be considered the most important phase.
When can you say your game is “in Beta”? One good way is if you feel like you might be able to hand your game to someone without having to explain anything about it. They can do a “blind playtest” (which doesn’t mean they’re blind, it just means that you don’t tell them anything before they play) and when they give feedback to you, you feel like they’re reacting to the right game. If you feel like your game still needs to be explained because you’re missing too much, then it’s probably still Alpha.