Approximants are speech sounds that involve the articulators approaching each other but not narrowly enough[1] nor with enough articulatory precision[2] to create turbulent airflow.
[3] This class is composed of sounds like [ɹ] (as in rest) and semivowels like [j] and [w] (as in yes and west, respectively), as well as lateral approximants like [l] (as in less).
In phonology, approximant is also a distinctive feature that encompasses all sonorants except nasals, including vowels, taps, and trills.
[6] Some approximants resemble vowels in acoustic and articulatory properties and the terms semivowel and glide are often used for these non-syllabic vowel-like segments.
[7] Vowels and their corresponding semivowels alternate in many languages depending on the phonological environment, or for grammatical reasons, as is the case with Indo-European ablaut.
[21] It is not often clear, however, whether such sequences involve a semivowel (a consonant) or a diphthong (a vowel), and in many cases, it may not be a meaningful distinction.
Although many languages have central vowels [ɨ, ʉ], which lie between back/velar [ɯ, u] and front/palatal [i, y], there are few cases of a corresponding approximant [ ȷ̈].
For example, the Spanish word ayuda ('help') features a palatal approximant that is pronounced as a fricative in emphatic speech.
However, the defining location is the side of the tongue, which only approaches the teeth, allowing free passage of air.
Fricative consonants are generally said to be the result of turbulent airflow at a place of articulation in the vocal tract.
[32] It is not clear if it is possible to describe voiceless approximants categorically as having laminar airflow (or cavity friction in Pike's terms) as a way of distinguishing them from fricatives.
Ball & Rahilly (1999) write that "the airflow for voiced approximants remains laminar (smooth), and does not become turbulent.
"[35] Voiceless approximants are treated as a phonetic category by (among others) Ladefoged & Maddieson (1996), Catford (1977), and Bickford & Floyd (2006).