A dental fricative (/ˈdentəl ˈfrɪkətɪv/), commonly known as a TH sound, is a fricative (a type of consonant sound) made with the tongue pushing up against the top front teeth. There are two of these in English; a voiced dental fricative (which is a voiced sound) and an unvoiced dental fricative (which is an unvoiced sound).
These are the only consonant sounds that are not part of the English core consonant sounds necessary to competently speak English.
Th-alveolarisation, Th-fronting, and Th-stopping are common realisations of this sound by learners as respectively (1) /s/, & /z/ (2) /f/ and /v/ (3) /t/ and /d/. Meanwhile, people may lisp sibilants to dental fricatives.