A nostril 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 nostrils of some whales are highly asymmetric, one nostril proceeding straight diagonally up to the blowhole, the other detouring to the back of the head.
Procellariiformes are distinguished from other birds by having tubular extensions of their nostrils.
In humans its is quite normal for just one nostril to be functioning at any one time. During the course of a day they will switch over approximately every four hours or so.