Here's what might be meant by "abstract vowels" and why it's not a typical term:
* Vowels as abstract sounds: Vowels are often described in terms of their position in the mouth (high/low, front/back) and tongue tension (tense/lax). These descriptions are abstract representations of the actual sounds produced.
* Vowel phonemes: In phonology, we talk about vowel phonemes. A phoneme is a basic sound unit of a language. Each phoneme can have different realizations (allophones) depending on the context. For example, the "a" sound in "cat" and "father" are technically different sounds, but they belong to the same phoneme. It's possible someone is using "abstract vowel" to refer to vowel phonemes.
* Vowel symbols: In writing, we use vowel symbols to represent sounds. These symbols are abstract representations of sounds and don't directly correspond to how the sound is produced.
If you can provide more context or an example of how "abstract vowels" is being used, I might be able to give you a more precise answer.