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 above the symbol for the sound to be nasalized: [ã] is the nasalized equivalent of [a], and [ṽ] is the nasalized equivalent of [v]. A subscript diacritic [ą], called an ogonek or nosinė, is sometimes 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 vowel
Many languages have nasal vowels to different degrees, but only a minority of world languages around the world have nasal vowels as contrasting phonemes. That is the case, among others, of French, Portuguese, Hindustani, Nepali, Breton, Gheg Albanian, Hmong, Hokkien, Yoruba, and Cherokee. Those nasal vowels contrast with their corresponding oral vowels. Nasality is usually seen as a binary feature, although surface variation in different degrees of nasality caused by neighboring nasal consonants has been observed.
There are occasional languages, such as in Palantla Chinantec, where vowels seem to exhibit three contrastive degrees of nasality: oral e.g. [e] vs lightly nasalized [ẽ] vs heavily nasalized [e͌], although Ladefoged and Maddieson believe that the lightly nasalized vowels are best described as oro-nasal diphthongs. Note that Ladefoged and Maddieson's transcription of heavy nasalization with a double tilde might be confused with the extIPA adoption of that diacritic for velopharyngeal frication.
Nasal consonant
By far the most common nasal sounds are nasal consonants such as [m], [n] or [ŋ]. Most nasal consonants are occlusives, and airflow through the mouth is blocked and redirected through the nose. Their oral counterparts are the stops.
Nasalized versions of other consonant sounds also exist but are much rarer than either nasal occlusives or nasal vowels.