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In phonetics, nasalization is the production of a sound while the velum is lowered, so that some air escapes through the nose during the production of the sound by the mouth. The effect is as if an [n] sound were produced simultaneously with the oral sound.

In the International Phonetic Alphabet nasalization is indicated by printing a tilde above the symbol for the sound to be nasalized: [ã] is the nasalized equivalent of [a], and [ṽ] is the nasalized equivalent of [v]. An older IPA subscript diacritic [ą], called an ogonek, is still seen, especially when the vowel bears tone marks that would interfer with the superscript tilde. [ą̂], for example, is more legible in most fonts than [ã̂].

Main article: Nasal vowel

The most common nasalized sounds are nasal vowels, found in French, Polish, Portuguese, and the Texas "twang"^{[citation needed]}. There are occasional cases where vowels show contrasting degrees of nasality^{[citation needed]}.

However, there are also nasalized consonants which contrast with purely oral consonants. Some of the South Arabic languages have nasalized fricatives, such as [z̃], which sounds something like a simultaneous [n] and [z]. The sound written r in Mandarin has an odd history; for example, it has been borrowed into Japanese as both [z] and [n]. It seems likely that it was once a nasalized fricative, perhaps a palatal [ʝ̃]. In the Hupa velar nasal /ŋ/, the tongue often does not make full contact, resulting in a nasalized approximant, [ɰ̃]. This is cognate with a nasalized [j̃] in other Athabaskan languages. In Umbundu, phonemic [ṽ] contrasts with (allophonically) nasalized [w̃], and so is likely to be a true fricative rather than an approximant.

Phonologically speaking, nothing prevents from describing nasal stops such as /m/, /n/ or /ŋ/ as the nasalized counterpart of voiced oral stops. In theory, these nasal consonants could therefore perfectly be represented as, respectively, /b̃/, /d̃/ or /g̃/. The only reason why these nasal consonants have their own symbol is their frequency in the world's languages – in contrast, for example, with the nasal(ized) constrictives (e.g., [ṽ]). Nasal stops are called stops because airflow through the mouth is blocked, but air flows freely through the nose.

Besides nasalized oral fricatives, there are true nasal fricatives, called nareal fricatives, sometimes produced by people with speech defects. That is, the turbulence in the airflow characteristic of fricatives is produced not in the mouth but in the nasal passages. A tilde plus trema diacritic is used for this in the Extended IPA: [n͋] is an alveolar nareal fricative, with no airflow out of the mouth, while [v͋] is an oral fricative (a [v]) with simultaneous nareal frication. No known natural language makes use of nareal consonants.

Main article: Denasal

Nasalization may be lost over time. There are also denasal sounds, which sound like nasals spoken with a head cold, but these are not used in non-pathological speech.

• Nasal consonant
• Prenasalized stop
• Nasal release