A nostril (or naris, pl. nares) is one of the two channels of the nose, from the point where they bifurcate to the external opening. In birds and mammals, they contain branched bones or cartilages called turbinates, whose function is to warm air on inhalation and remove moisture on exhalation. Fish do not breathe through their noses, but they do have two small holes used for smelling which may be called nostrils.
The Procellariiformes are distinguished from other birds by having tubular extensions of their nostrils.
In humans, the nasal cycle is the normal ultradian cycle of each nostril's blood vessels becoming engorged in swelling, then shrinking. During the course of a day, they will switch over approximately every four hours or so, meaning that only one nostril is used at any one time.
The nostrils are separated by the septum. The septum can sometimes be deviated, causing one nostril to appear larger than the other. In extraordinary cases such as that of British television actress Danniella Westbrook, excessive use of cocaine can cause the septum to become disfigured or destroyed. In such an event, the two nostrils are no longer separated and form a single larger external opening.