TL;DR

There are 4 general types of testing, delta, alpha, beta, gamma,

• Delta is quick testing of just the changes.
• Alpha is testing by the company creating the software.
• Beta testing is the most commonly heard - it is testing by select customers
• Gamma testing is testing by a wide range of companies.

The terms are often used incorrectly and 'beta' is often a catch bucket for everything.

Names of types of testing:

alpha α-testing

Alfa testing is "only people directly associated with the company are testing, no customers". Alpha is the 1st letter in the Greek alphabet.

beta β-testing

Beta testing is easily the most commonly heard, Beta is 2nd letter in Greek Alphabet means "Select Customer testing". (often it is used for ANY customer testing, beta or gamma)

gamma γ-testing

Gamma testing, the 3rd letter in the Greek alphabet) meaning "Wide testing underway, not just 'select' people". Note that many companies CALL "Gamma" tests "Beta" because more people have heard of Beta testing than have heard of alpha or gamma testing. 

delta Δ-testing

Delta testing is testing ONLY changes. This word is not often used, and in practice is often (by us as well) incorrectly called Alpha testing.. Interestingly while Alpha, Beta, Gamma testing are done in that order, Delta (the 4th letter of the Greek alphabet) is usually done BEFORE or INSTEAD of alpha testing. Delta is in science 'the difference between' hence the "only differences testing". The first 3 take NO meaning from the Greek letters, Delta is the 1st one that takes meaning from the name of the letter.

epsilon ε through Ω,omega testing

I have never heard of Epsilon through Omega testing (the 5th through 25th letters of the Greek alphabet) in the context of software.

Final words

Many companies use the terms incorrectly. And they are more of a guideline than a rule. You might allow a tiny number of customers to participate in an alpha. You might have a 'wide beta' aka "gamma".

The most important part is - making sure software is tested!

Interesting side notes of no value whatsoever for EAM/CMMS, but my Greek professor would be happy:

For alpha, beta and gamma, the lower case Greek letters are used, α, β, γ if it is done by using the Greek letters. But for delta testing, the capital letter Δ is used, because if you saw the lower case Delta ð you probably would not know that it was the letter delta.

The word Alphabet is the Greek words alpha and bet(a) ... the 1st 2 letters of the Greek alphabet.

Why is z the last letter of the English alphabet when it is the 4th letter of the Greek alphabet?

Z is the last letter of the English alphabet because of its historical removal and late reintroduction to the Latin alphabet, Around 300 BC, the Roman Censor Appius Claudius Caecus removed Z from the Latin alphabet because the /z/ sound had shifted to /r/ (a process called rhotacism), making the letter redundant in native Latin words. Around 100BC Z was added back to the Latin alphabet to accommodate loanwords from Greek, and it was 'too late' to put it back in it's original spot so it was tagged on to the end.

Time to go α-test the next version of our software ...