Some people say that alter derives from αλλότε??ος (allotterοs ), which is the Aeolic form of αλλότ?ιος (allotrios ) [foreign, different, one as different from another]. See: http://ewonago.blogspot.com/
Some other say that alter is a combination of the Greek words άλλος + έτε?ος (allos+eteros) (other, different, oposite).
Finally, others say that alter is just the Latin word alter.
Is there another theory?
What do you think?
The best authority for these questions is the OED:
a. 14th c. Fr. altére-r (Pr. or It. alterar) ad. med.L. altera-re, f. alter other.
So it’s from a late Latin verb built upon alter.
As with so much Latinate vocabulary in English, this word got filtered through French first.
Neos, I appreciate that you have given consideration to my critique. This is good. Open mindness is extremely important.
I would like you to consider something else: what is the source for these interpretations of etymology? You say here that, perhaps there is one of two direct Greek origins for “alter,” for example, or a Latin origin. I insist (as I believe will the majority insist) that the Latin origin is the correct one.
How do I know this?
It is the source of my understanding. I can cite this reference from Webster (and countless others, including Latin dictionaries) that demonstrate the origin of “alter” :
*** http://www.merriam-webster.com/dictionary/alter
Main entry: al·ter
Pronunciation:
\ˈȯl-tər
Function:
verb
Inflected Form(s):
al·tered; al·ter·ing -t(ə-)riŋ
Etymology:
Middle English, from Medieval Latin alterare, from Latin alter other (of two); akin to Latin alius other — more at else
Date:
14th century
transitive verb
1 : to make different without changing into something else
2 : castrate, spay
intransitive verb
: to become different
This source and others confirm my “theory.” What references (published, mind you, and formal) confirm your theory?
Thank you for your constructive comments and your polite manners.
Best regard
John Neos
I am sorry but I can’t be polite to someone who makes a mockery of linguistics, shames Greeks and tells the world that there were people who had no word for i.e. “milk” until they got it from the Greeks I suppose. Or changed the word they had and adopted a word derived from the Greek.
Sure you can! 
You just have to try a little harder! lol Remember the old saying about the flies and the honey and vinegar?
Doesn’t really work in this case and, in any case, I really can’t. I lost my patience with such people long ago you see. It’s sad.
Actually, Irene, I think he might have been genuinely thanking me for being polite. I hope I was polite anyway. It wasn’t a sarcastic jibe at you, I don’t think.