A nasal vowel is a vowel that is produced with a lowering of the velum so that air escapes both through the nose as well as the mouth, such as the French vowel /ɑ̃/ (help·info). By contrast, oral vowels are vowels without the nasalization. As explained below, nasal vowels that are distinctive or obligatory are of far more linguistic importance than whether or not speakers of a language tend to nasalize vowels in some instances. Relatively similar languages in the same branch of a language family differ on this point quite frequently throughout the world, such as in Spanish and Portuguese.
In most languages, vowels that are adjacent to nasal consonants are produced partially or fully with a lowered velum in a natural process of assimilation and are therefore technically nasal, but few speakers would notice. That is the case in English: vowels preceding nasal consonants are nasalized, but there is no phonemic distinction between nasal and oral vowels (and all vowels are considered phonemically oral).
However, the words "huh?" and "uh-huh" are pronounced with a nasal vowel, as is the negative "unh-unh".
In French, by contrast, nasal vowels are phonemes distinct from oral vowels, and words can differ by this vowel quality. For example, the words beau /bo/ "beautiful" and bon /bõ/ "good" are a minimal pair that contrasts primarily the vowel nasalization, even if the /õ/ from bon is slightly more open.
Portuguese behaves similarly, with minimal pairs as tumba /tũba/ "tomb" and tuba /tuba/ "tuba", except /ũ/ and /u/ have the same openness. The language also allows nasal diphthongs that contrast with their oral counterparts, like the minimal pair pão /pãw̃/ [pɐ̃w̃] "bread" and pau /paw/ [paʊ] "stick".
Although there are French loanwords into English with nasal vowels like croissant, there is no expectation that an English speaker would nasalize the vowels to the same extent that French or Portuguese speakers do. Likewise, pronunciation keys in English dictionaries do not always indicate nasalization of French loanwords.
Nasalization may cause a vowel's articulation to shift. However, nasalization from the assimilation of a nasal consonant tends to cause a raising of vowel height; phonemically distinctive nasalization tends to lower the vowel.
A few languages, such as Palantla Chinantec, contrast lightly nasalized and heavily nasalized vowels. They may be contrasted in print by doubling the IPA diacritic for nasalization: ⟨ẽ⟩ vs ⟨ẽ̃⟩. Bickford & Floyd (2006) combine the tilde with the ogonek: ⟨ẽ⟩ vs ⟨ę̃⟩. (The ogonek is sometimes used in an otherwise IPA transcription to avoid conflict with tone diacritics above the vowels.)
Languages that are written in the Latin alphabet may indicate nasal vowels by a trailing silent n or m, as is the case in French, Portuguese, Lombard (central classic orthography), Bamana, or Yoruba. In other cases, they are indicated by diacritics: Portuguese also marks nasality with a tilde, ã, õ, before other vowels; Breton indicates a nasal vowel by a trailing n which can be made silent with a tilde, as in bezañ, "to be"; Polish, Navajo, and Elfdalian use a hook under the letter, called an ogonek, as in ą, ę.
Other languages may use a superscript n (aⁿ, eⁿ, ...), as in the Pe̍h-ōe-jī romanization of Southern Min. In the International Phonetic Alphabet, nasal vowels are denoted by a tilde over the symbol for the vowel, as in Portuguese.
The Nasta'liq script, used by Urdu, denotes nasalization by employing the Arabic letter ˂ن˃ nūn but removing the dot (˂ں˃), called nūn ghunna. Nasalized vowels occur in Classical Arabic but not in contemporary speech or Modern Standard Arabic. There is no orthographic way to denote the nasalization, but it is systematically taught as part of the essential rules of tajweed, used to read the Qur'an. Nasalization occurs in recitation, usually when a final ˂ن˃ nūn is followed by a ˂ي˃ yāʼ.
These languages use phonemic nasal vowels: