The Collaborative International Dictionary of English v.0.48:

Nostril \Nos"tril\, n. [OE. nosethril, nosethirl, AS.
   nos[thorn]yrl; nos for nosu nose + [thorn]yrel opening, hole,
   from [thorn]yrel pierced, for [thorn]yrhel, fr. purh through.
   [root]261. See Nose, and Through, and cf. Thrill.]
   [1913 Webster]
   1. (Anat.) One of the external openings of the nose, which
      give passage to the air breathed and to secretions from
      the nose and eyes; one of the anterior nares.
      [1913 Webster]

   Note: In sperm whales, porpoises, and allied animals, there
         is only one nostril, which is situated on the top of
         the head and called a spiracle.
         [1913 Webster]

   2. Perception; insight; acuteness. [Obs.]
      [1913 Webster]

            Methinks a man
            Of your sagacity and clear nostril should
            Have made another choice.             --B. Jonson.
      [1913 Webster]

WordNet (r) 3.0 (2006):

nostril
    n 1: either one of the two external openings to the nasal cavity
         in the nose [syn: nostril, anterior naris]