stair

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A series or flight of steps; a staircase.
• noun One of a flight of steps.

from The Century Dictionary.

• noun A step; a degree.
• noun One of a series of steps to mount by: as, a flight of stairs.
• noun A flight or succession of flights of steps, arranged one behind and above the other in such a way as to afford passage from a lower to a higher level, or vice versa: as, a winding stair; the back stair: often used in the plural in the same sense.

from the GNU version of the Collaborative International Dictionary of English.

• noun One step of a series for ascending or descending to a different level; -- commonly applied to those within a building.
• noun 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.
• noun in the basement or lower part of a house, where the servants are.
• noun the stairs which make the whole ascent of a story.
• noun a set or flight of stairs. -- pair, in this phrase, having its old meaning of a set. See Pair, n., 1.
• noun (Arch.) a single set of stairs, or section of a stairway, from one platform to the next.
• noun a rod, usually of metal, for holding a stair carpet to its place.
• noun See Upstairs in the Vocabulary.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A single step in a staircase.
• noun A series of steps, a staircase.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun support consisting of a place to rest the foot while ascending or descending a stairway