It's a combination of vowels articulated without pause, as in "mouse"
... consonant shift n : a set of regular changes in consonant articulation in 
the ... monosyllabic speech sound (as the vowel combination at the end of toy) 
that .... hiatus n : the occurrence of two vowel sounds without pause or 
intervening... [DOC] [TLE] Grammar And Language Glossary - Businessballs [PAR] Not all words which begin with 'a' are using the 'a' prefix in this way.