The Collaborative International Dictionary of English v.0.48:

off-season \off-season\ adj.
   of or relating to a season of less than maximum demand; as,
   off-season hotel rates are lower.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

off-season
    n 1: the season when travel is least active and rates are lowest
         [ant: high season, peak season]