a vowel-like sound occurring in consonantal positions in the same syllable with a true vowel, characterized by brief duration and rapid change from one position of articulation to another: the English glides (w) and (y) are semivowels
See semivowel in American Heritage Dictionary 4
noun
A sound that has the quality of one of the high vowels, as (ē) or (o͞o), and that functions as a consonant before or after vowels, as the initial sounds of yell and well and the final sounds of coy and cow. Also called glide.