[syn: downstairs, down the stairs, on a lower floor, below]
The Collaborative International Dictionary of English v.0.48:
downstair \downstair\, downstairs \downstairs\adj.
   on or of the lower floors of a building, especially the
   ground floor; as, the downstairs (or downstair phone; the
   house has no downstairs bathroom. Opposite of upstairs.
   [WordNet 1.5]
The Collaborative International Dictionary of English v.0.48:
downstairs \down"stairs\, adv.
   Down the stairs; to a lower floor; as, she headed downstairs
   as soon as she heard the horn.
   [1913 Webster +PJC]
WordNet (r) 3.0 (2006):
downstairs
    adv 1: on a floor below; "the tenants live downstairs" [syn:
           downstairs, down the stairs, on a lower floor,
           below] [ant: on a higher floor, up the stairs,
           upstairs]
    adj 1: on or of lower floors of a building; "the downstairs (or
           downstair) phone" [syn: downstairs, downstair] [ant:
           upstair, upstairs]