Source : Webster's Revised Unabridged Dictionary (1913)

Downstairs \Down"stairs\, adv.
   Down the stairs; to a lower floor. -- a. Below stairs; as, a
   downstairs room.

Source : WordNet®

downstairs
     adj : on or of lower floors of a building; "the downstairs (or
           downstair) phone" [syn: {downstair}] [ant: {upstairs}]
     adv : on a floor below; "the tenants live downstairs" [syn: {down
           the stairs}, {on a lower floor}, {below}] [ant: {upstairs}]