An obstruent is a plosive, a fricative, or an affricate, that is, any consonant where airflow through the vocal tract is obstructed either completely or at least enough to create turbulence.
Obstruent is a name referring to consonants (stops and fricatives) that are produced by narrowing or complete closure of the vocal tract; the air pressure is built up inside the cavity. Obstruents share the feature [-son].
Consonants that are not obstruents are sonorants. In many versions of distinctive feature theory, obstruents are represented with the minus-value of the feature [son] or [sonorant].
Utrecht Lexicon of Linguistics