Semivowel

Definition
The sounds /w,j,r,l/ are often called semivowels because their formant structures are like those for vowels and diphthongs. The /r/ and /l/ sounds produced in syllable final position can be prolonged as in 'car' or 'full' and sound much like vowels. When /w/ or /j/ are produced slowly enough, the vowels /u/ and /i/ can be heard.

Links
Utrecht Lexicon of Linguistics