Alpha and beta refer to early versions of software, before it's formally released. Alpha versions are tested internally by the software's makers or select volunteers and may still have significant bugs. Beta versions are closer to complete and more functional and are often tested by potential customers or the general public.
Alpha Versions of Software
Alpha, the first letter in the Greek alphabet, is used for the first testable version of a piece of software. The terms alpha and beta are said to have originated at IBM.
Alpha testing generally refers to internal testing of software by the organization developing it, though some organizations recruit outside volunteers to test alpha software as well.
At the alpha stage, the software can have significant defects or missing features, incomplete documentation and may even require a special computing environment to get it to run at all.
Alpha testing may go through several iterations, and versions may add and subtract working features while developers fix bugs and respond to test feedback.
Once software is reasonably functional and close to ready for release, it moves from alpha testing to beta testing.
Beta versions of software are generally much closer to release-ready products than alpha versions. They're often tested by existing or potential customers of the software's makers or even the public at large.
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Beta software is usually "feature complete" -- meaning the full set of functionality expected for the finished product is already in place -- and has fewer defects than alpha software. Documentation is also usually complete or close to it, and the software is generally ready to run in the computing environment it will run in after deployment.
While beta tests are traditionally shorter than alpha tests, since they're designed to catch the last issues before a software is released for sale or public distribution, some modern Web software remains in a beta stage for months or even years, as developers solicit feedback, bug reports and suggestions from users and try out new features.