Definition - A type of consonant that is created by blocking the mouth with the tongue or lips, lowering the soft palate, and allowing the air to come out through the nose.

1. Vowels can be nasalised by allowing some air to come out through the nose and mouth at the same time.

Example -
The [m] in mouse and the [n] in nose are nasals.

Etymology -
The word derives via French from the Latin nasus, nose.
Note: Ultimately it derives from the PIE *nas- (cf. the Sanskrit nasa, the Old Church Slavonic nasu, and the Lithuanian nosis).

Oxford English Dictionary -
Its first citation is from 1669:
"In attempting to pronounce these two Consonants, as likewise the Nasals, and some of the vowels."
(Holder Elem. Speech 59 )

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