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.
In humans, the nasal cycle means that during the course of a day, the nostrils will switch over approximately every four hours or so, meaning that only one nostril is used at any one time.