The semivowels are the sounds [w], [y], and [h]. They are like vowels in that they are pronounced smoothly and without friction or interruption, and they are like consonants in that they occur at the margins of syllables rather than at the peak – as in way, yea, hay. In the field of linguistics called “distinctive feature analysis”, among the several contrasting features that are recognized are “vocalic” and “consonantal,” which have to do with the manner of articulation. A given sound either has or does not have each feature, represented usually with plusses and minuses. Vowels and consonant sounds are described as follows:
+ Vocalic and – Consonantal = Vowels
– Vocalic and + Consonantal = Consonants
But there are two other possible combinations of the vocalic and consonantal features, describing two other groups of sounds, liquids and semivowels:
+ Vocalic and + Consonantal = Liquids [l] and [r]
– Vocalic and – Consonantal = Semivowels [h], [w], [y]
In terms of manner of articulation, then, semivowels are just the opposite of liquids. And if you think about how they function in syllables, they are just the opposite of syllabic consonants, which can function like a vowel as the peak of a syllable, as at the end of button and bottle.