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]

Moby Thesaurus II by Grady Ward, 1.0:

52 Moby Thesaurus words for "nostril":
   air duct, air hole, air passage, air shaft, air tube, airway,
   antlia, beak, beezer, bill, blowhole, breathing hole, bugle, conk,
   louver, louverwork, muffle, muzzle, nares, naris, neb, nib, nose,
   nostrils, nozzle, olfactories, olfactory area, olfactory cell,
   olfactory nerves, olfactory organ, olfactory pit, pecker,
   proboscis, rhinarium, rostrum, schnozzle, shaft, smeller, snoot,
   snout, spilehole, spiracle, touchhole, transom, trunk, vent,
   ventage, venthole, ventiduct, ventilating shaft, ventilator,
   wind tunnel