From Infogalactic: the planetary knowledge core
A continuant is a speech sound produced with an incomplete closure of the oral vocal tract. Among consonants, there are two broad categories of continuants: fricatives and approximants. Approximants are sometimes called "frictionless continuants".
Compare sonorant (resonant), which includes approximants and nasals but not fricatives. Both terms technically include vowels, but are frequently restricted to the context of consonants.