Pharr's Homeric Greek - Lesson 1

503. Semi-vowel describes a vowel that can also be pronounced like a consonant. In earlier Greek, iota could sound like the 'i' in 'onion'; upsilon like the 'u' in 'persuade' - ('y' and 'w' respectively). Because of various sound changes over time, this semi-vocalic or consonantal pronunciation of iota and upsilon tended to disappear.