Source : Webster's Revised Unabridged Dictionary (1913)

Stair \Stair\, n. [OE. steir, steyer, AS. st?ger, from ?igan to
   ascend, rise. [root]164. See {Sty} to ascend.]
   1. One step of a series for ascending or descending to a
      different level; -- commonly applied to those within a
      building.

   2. A series of steps, as for passing from one story of a
      house to another; -- commonly used in the plural; but
      originally used in the singular only. ``I a winding stair
      found.'' --Chaucer's Dream.

   {Below stairs}, in the basement or lower part of a house,
      where the servants are.

   {Flight of stairs}, the stairs which make the whole ascent of
      a story.

   {Pair of stairs}, a set or flight of stairs. -- pair, in this
      phrase, having its old meaning of a set. See {Pair}, n.,
      1.

   {Run of stars} (Arch.), a single set of stairs, or section of
      a stairway, from one platform to the next.

   {Stair rod}, a rod, usually of metal, for holding a stair
      carpet to its place.

   {Up stairs}. See {Upstairs} in the Vocabulary.

Source : WordNet®

flight of stairs
     n : a stairway (set of steps) between one floor or landing and
         the next [syn: {flight}, {flight of steps}]