Well, let’s because it was not a grammatical phenomenon, hence no name.
Now, there is an underlying linguistic phenomenon: a diphthong. I believe that the Æ sound was considered the main diphthong, and was referred to as ‘diphthongus’. But that’s the sound, not the writing.
There is a very good case to be made for view that the spelling change from '
AI' to '
Æ' at the start of the Classical period was not arbitray, but a reflexion of a change in pronunciation.
http://www.bhthom.org/missatridentina/phonemes.htm
'
Æ' is to be regarded as a single, independent letter representing a single pure and simple vowel. The shape of the letter is intended to indicate that its sound is intermediate between that of '
a' and that of '
e'. Regarded as either a fronted '
a' or an open '
e', it should always sound like '
æ' in '
æroplane', and always be of long duration. It is always long for the historical reason that it resulted from the simplification of an Old Latin diphthong which, until about two centuries before Christ, was spelt '
AI' and pronounced like '
ai' in 'samur
ai'. Some advocates of 'Classical' pronunciation insist on pronouncing it in this preclassical way despite the change of spelling, citing perhaps the fact that Classical Latin writers used '
Æ' to translitterate the similar diphthong which continued to be written as '
AI' in Greek. The true explanation for this, however, is that this ancient diphthong had undergone almost exactly the same change in most Greek dialects, but that Greek spelling had solidify'd at an earlier time, and was thus more reluctant to reflect such changes.