In phonetics, nasalization (or nasalisation) 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. An archetypal nasal sound is [n].

In the International Phonetic Alphabet nasalization is indicated by printing a tilde diacritic U+0303 ◌̃ combining tilde (HTML: ̃) 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 interfere with the superscript tilde. For example, [ą̄ ą́ ą̀ ą̂ ą̌] are more legible in most fonts than [ã̄ ã́ ã̀ ã̂ ã̌].

Nasal vowels are found in many European languages, such as French, Portuguese, Breton, Polish. In these, as well as and languages found in other language families outside Europe, nasal vowels contrast with oral vowels. Many languages, however, only have oral vowels.