A semivowel is a sound that acts as both a vowel and a consonant. The two main semivowels in English, /w/ and /j/, as in wet and yet, are known as "glides", but there are others, such as /r/ and /l/, known as "liquids".
Glides occur before or between vocalic nuclei.
- what - west - war;
- yacht - yes - your;
Liquids occur after vocalic nuclei and before consonants.
- full - pool
- "semivowel" Collins English Dictionary
- Lawrence J. Raphael, Gloria J. Borden, Katherine S. Harris (2007) Speech Science Primer: Physiology, Acoustics, And Perception of Speech, p. 133. Lippincott Williams & Wilkins. At Google Books.