Here are some possibilities of what you might be referring to:
* Constant vowel sound: This could mean a vowel sound that is consistently pronounced in the same way, regardless of surrounding sounds. However, this is not a common term. Vowel sounds are typically influenced by surrounding sounds (a phenomenon called phonetic assimilation).
* Vowel harmony: This is a phenomenon where vowels in a word or phrase tend to be similar in their qualities (e.g., front vs. back, high vs. low). This is more commonly described as "vowel harmony", not "constant vowel".
* Vowel reduction: This is a phenomenon where vowels in unstressed syllables become less distinct or change their quality, often becoming more like the schwa sound. This is the opposite of a "constant vowel" as it describes a vowel sound that changes based on the context.
Could you provide more context or clarify your question? For example, where did you come across the term "constant vowel"? This might help me understand what you're trying to ask.