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 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).
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. The correlation between semivowels and vowels is strong enough that cross-language differences between semivowels correspond with the differences between their related vowels.
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. Similarly, languages often avoid configurations where a semivowel precedes its corresponding vowel. A number of phoneticians distinguish between semivowels and approximants by their location in a syllable. Although he uses the terms interchangeably, Montreuil (2004:104) remarks that, for example, the final glides of English par and buy differ from French par ('through') and baille ('tub') in that, in the latter pair, the approximants appear in the syllable coda, whereas, in the former, they appear in the syllable nucleus. This means that opaque (if not minimal) contrasts can occur in languages like Italian (with the i-like sound of piede 'foot', appearing in the nucleus: [ˈpi̯ɛˑde], and that of piano 'slow', appearing in the syllable onset: [ˈpjaˑno]) and Spanish (with a near minimal pair being abyecto [aβˈjekto] 'abject' and abierto [aˈβi̯erto] 'opened').
In articulation and often diachronically, palatal approximants correspond to front vowels, velar approximants to back vowels, and labialized approximants to rounded vowels. In American English, the rhotic approximant corresponds to the rhotic vowel. This can create alternations (as shown in the above table).
In addition to alternations, glides can be inserted to the left or the right of their corresponding vowels when they occur next to a hiatus. For example, in Ukrainian, medial /i/ triggers the formation of an inserted [j] that acts as a syllable onset so that when the affix /-ist/ is added to футбол ('football') to make футболіст 'football player', it is pronounced [futbo̞ˈlist], but маоїст ('Maoist'), with the same affix, is pronounced [mao̞ˈjist] with a glide. Dutch for many speakers has a similar process that extends to mid vowels:
Non-high semivowels also occur. In colloquial Nepali speech, a process of glide-formation occurs, where one of two adjacent vowels becomes non-syllabic; the process includes mid vowels so that [dʱo̯a] ('cause to wish') features a non-syllabic mid vowel. Spanish features a similar process and even nonsyllabic /a/ can occur so that ahorita ('right away') is pronounced [a̯o̞ˈɾita]. 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]. One is in the Korean diphthong [ ȷ̈i] or [ɨ̯i] though it is more frequently analyzed as velar (as in the table above), and Mapudungun may be another, with three high vowel sounds, /i/, /u/, /ɨ/ and three corresponding consonants, /j/, and /w/, and a third one is often described as a voiced unrounded velar fricative; some texts note a correspondence between this approximant and /ɨ/ that is parallel to /j/–/i/ and /w/–/u/. An example is liq /ˈliɣ/ ([ˈliɨ̯]?) ('white').
In addition to less turbulence, approximants also differ from fricatives in the precision required to produce them. When emphasized, approximants may be slightly fricated (that is, the airstream may become slightly turbulent), which is reminiscent of fricatives. For example, the Spanish word ayuda ('help') features a palatal approximant that is pronounced as a fricative in emphatic speech. Spanish can be analyzed as having a meaningful distinction between fricative, approximant, and intermediate /ʝ ʝ˕ j/. However, such frication is generally slight and intermittent, unlike the strong turbulence of fricative consonants.
For places of articulation further back in the mouth, languages do not contrast voiced fricatives and approximants. Therefore, the IPA allows the symbols for the voiced fricatives to double for the approximants, with or without a lowering diacritic.
Occasionally, the glottal "fricatives" are called approximants, since [h] typically has no more frication than voiceless approximants, but they are often phonations of the glottis without any accompanying manner or place of articulation.
In lateral approximants, the center of tongue makes solid contact with the roof of the mouth. However, the defining location is the side of the tongue, which only approaches the teeth, allowing free passage of air.
Voiceless approximants are not recognized by all phoneticians as a discrete phonetic category. There are problems in distinguishing voiceless approximants from voiceless fricatives.
Fricative consonants are generally said to be the result of turbulent airflow at a place of articulation in the vocal tract. However, an audible voiceless sound may be made without this turbulent airflow: Pike (1943) makes a distinction between "local friction" (as in [s] or [z]) and "cavity friction" (as in voiceless vowels like [ḁ] and [ɔ̥]). More recent research distinguishes between "turbulent" and "laminar" airflow in the vocal tract. 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. Voiceless approximants are rare in the languages of the world, but when they do occur the airflow is usually somewhat turbulent." Audible voiceless sounds may also be produced by means of turbulent airflow at the glottis, as in [h]; in such a case, it is possible to articulate an audible voiceless sound without the production of local friction at a supraglottal constriction. Catford (1977) describes such sounds, but classes them as sonorants.
Voiceless approximants are rarely if ever distinguished phonemically from voiceless fricatives in the sound system of a language. Clark & Yallop (1995) discuss the issue and conclude "In practice, it is difficult to distinguish between a voiceless approximant and a voiceless fricative at the same place of articulation ... there is no evidence that any language in the world makes such a distinction crucial."
Voiceless approximants are treated as a phonetic category by (among others) Ladefoged & Maddieson (1996), Catford (1977), and Bickford & Floyd (2006). However, the term voiceless approximant is seen by some phoneticians as controversial. It has been pointed out that if approximant is defined as a speech sound that involves the articulators approaching each other but not narrowly enough to create turbulent airflow, then it is difficult to see how a voiceless approximant could be audible. As John C. Wells puts it in his blog, "voiceless approximants are by definition inaudible ... If there's no friction and no voicing, there's nothing to hear." A similar point is made in relation to frictionless continuants by O'Connor (1973): "There are no voiceless frictionless continuants because this would imply silence; the voiceless counterpart of the frictionless continuant is the voiceless fricative." Ohala & Solé (2010) argue that the increased airflow arising from voicelessness alone makes a voiceless continuant a fricative, even if lacking a greater constriction in the oral cavity than a voiced approximant.
Ladefoged & Maddieson (1996) argue that Burmese and Standard Tibetan have voiceless lateral approximants [l̥] and Navajo and Zulu voiceless lateral fricatives [ɬ], but also say that "in other cases it is difficult to decide whether a voiceless lateral should be described as an approximant or a fricative". Asu, Nolan & Schötz (2015) compared voiceless laterals in Estonian Swedish, Icelandic, and Welsh and found that Welsh-speakers consistently used [ɬ], that Icelandic-speakers consistently used [l̥], and that speakers of Estonian Swedish varied in their pronunciation. They conclude that there is "a range of variants within voiceless laterals, rather than a categorical split between lateral fricatives and voiceless approximant laterals".
(Not to be confused with 'nasal continuant', which is a synonym for nasal consonant)
What are transcribed as nasal approximants may include non-syllabic elements of nasal vowels or diphthongs.