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
1. One step of a series for ascending or descending to a
different level; -- commonly applied to those within a
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
Pair of stairs, a set or flight of stairs. -- pair, in this
phrase, having its old meaning of a set. See Pair, n.,
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.