Approximants are speech sounds that involve the articulators approaching each other but not narrowly enough nor with enough articulatory precision to create turbulent airflow. Therefore, approximants fall between fricatives, which do produce a turbulent airstream, and vowels, which produce no turbulence. This class of sounds includes lateral approximants like (as in less), non-lateral approximants like (as in rest), and semivowels like and (as in yes and west, respectively).
Before Peter Ladefoged coined the term "approximant" in the 1960s the term "frictionless continuant" referred to non-lateral approximants.
Read more about Approximant Consonant: Semivowels, Approximants Versus Fricatives, Central Approximants, Lateral Approximants, Coarticulated Approximants With Dedicated IPA Symbols, Voiceless Approximants