Semi-vowels, aka semi-consonants, are sounds pronounced somewhere between vowels and consonants. (Très logique, n’est-ce pas ?) They’re created by partially obstructing the passage of air through the mouth, and can also be called glides or approximants.
French has three semi-vowels, each of which corresponds phonetically to a normal, "pure" vowel:
Characteristics of a semi-vowel
- Phonetically equivalent to a pure vowel
- Creates a diphthong with the vowel it precedes or follows
- Cannot be a syllable on its own
See IPA – vowels for details on how to pronounce and spell these sounds.