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 [?] (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).
Before Peter Ladefoged coined the term "approximant" in the 1960s, the term "frictionless continuant" referred to non-lateral approximants.
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: ['pide], and that of piano 'slow', appearing in the syllable onset: ['pja?no]) and Spanish (with a near minimal pair being abyecto[a?'jekto] 'abject' and abierto[a'?i?erto] 'opened').
^* Because of the articulatory complexities of the American English rhotic, there is some variation in its phonetic description. A transcription with the IPA character for an alveolar approximant ([?]) is common, though the sound is more postalveolar. Actual retroflexion may occur as well and both occur as variations of the same sound. However, Catford (1988:161f) makes a distinction between the vowels of American English (which he calls "rhotacized") and vowels with "retroflexion" such as those that appear in Badaga; Trask (1996:310), on the other hand, labels both as r-colored and notes that both have a lowered third formant.
^** Because the vowels [i ?] are articulated with spread lips, spreading is implied for their approximant analogues, [j ?]. However, these sounds generally have little or no lip-spreading. The fricative letters with a lowering diacritic, ⟨⟩, may therefore be justified for a neutral articulation between spread [j ?] and rounded [? w].
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 ('football') to make 'football player', it is pronounced [futbo?'list], but ('Maoist'), with the same affix, is pronounced [mao?'jist] with a glide.Dutch for many speakers has a similar process that extends to mid vowels:
Similarly, vowels can be inserted next to their corresponding glide in certain phonetic environments. Sievers' law describes this behaviour for Germanic.
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 [d?o?a] ('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 [a?o?'?ita]. 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[?, ?], which lie between back/velar [?, u] and front/palatal [i, y], there are few cases of a corresponding approximant [ ]. One is in the Korean diphthong [ i] or [i] 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/, /?/ 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 /?/ that is parallel to /j/-/i/ and /w/-/u/. An example is liq/'li?/ (['li]?) ('white').
Approximants versus fricatives
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 /? j/. However, such frication is generally slight and intermittent, unlike the strong turbulence of fricative consonants.
Because voicelessness has comparatively reduced resistance to air flow from the lungs, the increased air flow creates more turbulence, making acoustic distinctions between voiceless approximants (which are extremely rare cross-linguistically) and voiceless fricatives difficult. This is why, for example, no language is known to contrast the voiceless labialized velar approximant [w?] (also transcribed with the special letter ⟨?⟩) with a voiceless labialized velar fricative [x?]. Similarly, Standard Tibetan has a voiceless lateral approximant, [l?], and Welsh has a voiceless lateral fricative [?], but the distinction is not always clear from descriptions of these languages. Again, no language is known to contrast the two.Iaai is reported to have an unusually large number of voiceless approximants, with /l? w?/.
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.
Voiceless approximants are rarely distinguished from voiceless fricatives. Iaai has an unusually large number of them, with /l? w?/ contrasting with /l ? w/ (as well as a large number of voiceless nasals). Attested voiceless approximants are:
^ abThere is dialectal and allophonic variation in the realization of /?/. For speakers who realize it as [?], Rubach (2002:683) postulates an additional rule that changes any occurrence of [w] from glide insertion into [?].
^ abThere have been repeated requests that the IPA created dedicated symbols for  and [ð?] - typically modifications of the base letters such as turned??? and ?ð? or reversed ⟨?⟩ and ⟨ð⟩ - but so far the IPA has deemed that there is insufficient need for them.
^Bickford & Floyd (2006), augmented by sources at individual articles for the glottal approximants
Catford, J. C (1988), A Practical Introduction to Phonetics, Oxford University Press
Delattre, P.; Freeman, D.C. (1968), "A dialect study of American R's by x-ray motion picture", Linguistics, 44: 29-68
Hall, T. A. (2007), "Segmental features", in de Lacy, Paul (ed.), The Cambridge Handbook of Phonology, Cambridge University Press, pp. 311-334, ISBN978-0-521-84879-4
Hallé, Pierre A.; Best, Catherine T.; Levitt, Andrea; Andrea (1999), "Phonetic vs. phonological influences on French listeners' perception of American English approximants", Journal of Phonetics, 27 (3): 281-306, doi:10.1006/jpho.1999.0097