The order of releases, much like the order of army platoons or many
other labled item follows the greek alphabet.
We see this commonly in:
Science: Alpha males in animal packs
From my own knowledge
chemistry: "To indicate the position of substituting atoms or groups
in certain compounds; as, [beta]-naphthol."
Astrology: "To designate some bright star, usually the second
brightest, of a constellation, as, [beta] Aurig[ae]."
Webster's Revised Unabridged Dictionary (1913)
Under the word "Beta"
"hardware or software systems often go through two stages of release
testing: Alpha (in-house) and Beta (out-house?)."
"This term derives from early 1960s terminology for product cycle
checkpoints, first used at IBM but later standard throughout the
industry. Alpha Test was the unit, module, or component test phase;
Beta Test was initial system test. These themselves came from earlier
A- and B-tests for hardware. The A-test was a feasibility and
manufacturability evaluation done before any commitment to design and
development. The B-test was a demonstration that the engineering model
functioned as specified. The C-test (corresponding to today's beta)
was the B-test performed on early samples of the production design,
and the D test was the C test repeated after the model had been in
production a while."
"The Jargon File 4.1.0"
I hope this clears up any questions as to why we use alpha and beta
for software releases, as well as the source of these terms. If you
have any questions please make a request for clarification before
rating this question.