Beta
Beta is short for "Beta test", which is the common name for software testing that has expanded beyond internal "alpha testing". It generally involves recruiting outside people to use the software in order to improve the chances that an error will be found before release and also to stress test the application with a significantly higher number of users.
In terms of PlanetSide, it refers to the beta test period in the approximately two months before the game was released in May, 2003. Many veterans of the game will state that they have "played since beta," indicating that they have been participating since before the official release of the game. This period may also be further distinguished as consisting of "closed beta", when only those invited could participate in the testing, and "open beta", when anyone could sign up to test.