Sometimes a build known as pre-alpha is issued, before the release of
an alpha or beta. In contrast to alpha and beta versions, the pre-alpha is not
"feature complete". When it is used, it refers to all activities performed
during the software project prior to software testing. These activities can
include requirements analysis, software design, software development and unit
testing.
In Open Source world, there are several types of pre-alpha versions.
Milestone versions include specific sets of functionality and are released
as soon as the functionality is complete. Nightly builds are versions
that are usually automatically checked out from the revision control system and
built, typically over night; these versions allow the testers to test the
recently implemented functionality immediately, and find the new bugs.