Approximant

An approximant is a type of consonant sound in phonetics and phonology characterized by a narrowing of the vocal tract without producing the turbulent airflow that defines a fricative. Approximants are sonorants, meaning they are typically voiced and share some phonetic properties with vowels, including relatively open articulation and little noisy frication. They contrast with stops and fricatives in that they neither fully close the vocal tract nor generate substantial turbulent noise.

In many languages the main members of the approximant class are the liquids and glides: the alveolar lateral approximant [l], the alveolar approximant [ɹ] (often represented as “r”), the palatal approximant [j] (the y-sound in yes), and the labio-velar approximant [w] (the w-sound in wet). Additional approximants can occur in various languages, such as the velar [ɰ] or other rhotic varieties. The glides [j] and [w] are frequently called semivowels and often function as part of syllable structures, while other approximants may occur as consonantal segments or, in some languages, as syllabic vowels.

In linguistic analysis, approximants are typically treated as a subset of sonorants and are described by their place of articulation and voicing. They often participate in phonotactics with vowels and semivowels and can exhibit language-specific allophony or voicing contrasts.

In mathematics and numerical analysis, an approximant refers to a function or expression that closely estimates another function. Notable examples include Padé approximants and various polynomial or rational approximants used to approximate functions locally or over an interval.

See also: semivowel, vowel, phonetics, Padé approximant.