Source : Webster's Revised Unabridged Dictionary (1913)

Narrow \Nar"row\, a. [Compar. {Narrower}; superl. {Narrowest}.]
   [OE. narwe, naru, AS. nearu; akin to OS. naru, naro.]
   1. Of little breadth; not wide or broad; having little
      distance from side to side; as, a narrow board; a narrow
      street; a narrow hem.

            Hath passed in safety through the narrow seas.
                                                  --Shak.

   2. Of little extent; very limited; circumscribed.

            The Jews were but a small nation, and confined to a
            narrow compass in the world.          --Bp. Wilkins.

   3. Having but a little margin; having barely sufficient
      space, time, or number, etc.; close; near; -- with special
      reference to some peril or misfortune; as, a narrow shot;
      a narrow escape; a narrow majority. --Dryden.

   4. Limited as to means; straitened; pinching; as, narrow
      circumstances.

   5. Contracted; of limited scope; illiberal; bigoted; as, a
      narrow mind; narrow views. ``A narrow understanding.''
      --Macaulay.

   6. Parsimonious; niggardly; covetous; selfish.

            A very narrow and stinted charity.    --Smalridge.

   7. Scrutinizing in detail; close; accurate; exact.

            But first with narrow search I must walk round This
            garden, and no corner leave unspied.  --Milton.

   8. (Phon.) Formed (as a vowel) by a close position of some
      part of the tongue in relation to the palate; or
      (according to Bell) by a tense condition of the pharynx;
      -- distinguished from wide; as [=e] ([=e]ve) and [=oo]
      (f[=oo]d), etc., from [i^] ([i^]ll) and [oo^] (f[oo^]t),
      etc. See Guide to Pronunciation, [sect] 13.

   Note: Narrow is not unfrequently prefixed to words,
         especially to participles and adjectives, forming
         compounds of obvious signification; as,
         narrow-bordered, narrow-brimmed, narrow-breasted,
         narrow-edged, narrow-faced, narrow-headed,
         narrow-leaved, narrow-pointed, narrow-souled,
         narrow-sphered, etc.

   {Narrow gauge}. (Railroad) See Note under {Gauge}, n., 6.

Source : WordNet®

narrow gauge
     n : a railroad track (or its width) narrower than the standard
         56.5 inches