I looked up apocope on the web and found that it is the omission of a letter or syllable at the end of a word. So apocope wouldn’t be quite right. Syncope, which is the omission of letters or syllables from the middle of a word, would be technically correct, but I was thinking about a term that dealt more with vowels and how some vowels were dropped from some words in their journeys from Old English to Middle English and Middle English to Modern English. Unfortunately, I think the term I’m looking for is in the one book I sold back to the book store in college