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 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).

Before Peter Ladefoged coined the term approximant in the 1960s, the terms frictionless continuant and semivowel were used to refer to non-lateral approximants.

In phonology, approximant is also a distinctive feature that encompasses all sonorants except nasals, including vowels, taps, and trills.

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 'plane', 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:

Similarly, vowels can be inserted next to their corresponding glide in certain phonetic environments. Sievers' law describes this behaviour for Germanic.