WordNet (r) 3.0 (2006):

side door
    n 1: an exterior door at one side of a building [syn: side
         door, side entrance]

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "side door":
   French door, archway, back door, back road, back stairs,
   back street, back way, barway, bolt-hole, bulkhead, by-lane,
   bypass, bypath, byroad, bystreet, byway, carriage entrance,
   cellar door, cellarway, covert way, detour, door, doorjamb,
   doorpost, doorway, escalier derobe, escape hatch, escape route,
   front door, gate, gatepost, gateway, hatch, hatchway, lintel,
   porch, portal, porte cochere, postern, propylaeum, pylon,
   roundabout way, scuttle, secret exit, secret passage,
   secret staircase, side road, side street, stile, storm door,
   threshold, tollgate, trap, trap door, turnpike, turnstile,
   underground, underground railroad, underground route