Or, also from Wiki:
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Alpha software is software that is not thoroughly tested by the developer before it is released to customers. Alpha software may contain serious errors, and any resulting instability could cause crashes or data loss.
[2] Alpha software may not contain all of the features that are planned for the final version.
[3] In general, external availability of alpha software is uncommon in
proprietary software, while
open source software often has publicly available alpha versions. The alpha phase usually ends with a
feature freeze, indicating that no more features will be added to the software. At this time, the software is said to be
feature complete. A beta test is carried out following
acceptance testing at the supplier's site (alpha test) and immediately prior to general release of the software as a product.
[4]
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And key gameplay functions are implemented, but far for done, and some gameplay features are not present, ergo, alpha.