Page "Semivowel" Paragraph 1

Semivowels form a subclass of approximants.

Although " semivowel " and " approximant " are sometimes treated as synonymous, most authors agree that not all approximants are semivowels, although the exact details may vary from author to author.

For example, don't consider the labiodental approximant to be a semivowel, while proposes that it should be considered one.