The Collaborative International Dictionary of English v.0.48:

Indoors \In"doors`\, adv.
   Within the house; -- sometimes separated, in doors.
   [1913 Webster]

WordNet (r) 3.0 (2006):

indoors
    adv 1: within a building; "in winter we play inside" [syn:
           inside, indoors] [ant: alfresco, out of doors,
           outdoors, outside]