The Collaborative International Dictionary of English v.0.48:

Stair \Stair\ (st[^a]r), n. [OE. steir, steyer, AS. st[=ae]ger,
   from st[imac]gan 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.
      [1913 Webster]

   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.
      [1913 Webster]

   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 stairs (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.
      [1913 Webster]