An ELT Glossary : Approximants / Semi vowels
Approximants are speech sounds which are made by bringing the articulators further towards each other than is done in the case of vowels (where the airflow from the lungs is unimpeded), but not as close as in the case of consonants - where there is always some sort of impediment of the airflow. These sounds allow still the air to pass without impediment. However, in words, they are used as consonants. For example /j/ in "yet" could be replaced with another consonant (bet, set met, pet etc) but not with a vowel.
They are therefore neither true vowels nor true consonants - hence the alternative name semi-vowel.
The English approximants are /r/ /l/ /w/ and /j/.