‘Beta’ is a standard term to denote a milestone release during production in which game functionality is included and optimised (but may have bugs), game content is finished (but may have some implementation errors), and which is considered nearly complete. Beta represents the sum total of what the game will be, and content or functionality changes beyond beta are usually considered to be outside the framework of a publishing contract (called ‘change control’).
Beta are preceded by prototype, pre-alpha, (sometimes) vertical slice and alpha milestones. They are followed by release candidate and gold master milestones.
Online publishers often release beta software to the public as what is called an ‘open beta’. Open betas invite customers to play the game, report any issues that they find and participate in the community surrounding the game ahead of launch. Open betas are also often a great way to engage players with a marketing story early on.