The Collaborative International Dictionary of English v.0.48:

Overtime \O"ver*time`\, n.
   1. Time beyond, or in excess of, a limit; esp., extra working
      time.
      [1913 Webster]

   2. (Sports) An extra period of time provided to play a game,
      beyond the end of the normal period allowed for the game,
      for the purpose of resolving a tie score; as, the team won
      in overtime.
      [PJC]

   sudden death overtime an overtime[2] in which the first
      team to score wins the game; -- contrasted with normal
      overtime[2], which is a fixed period of time during which
      either team may score as often as they can.
      [PJC]

WordNet (r) 3.0 (2006):

overtime
    adv 1: beyond the regular time; "she often has to work overtime"
    n 1: work done in addition to regular working hours
    2: playing time beyond regulation, to break a tie [syn:
       overtime, extra time] [ant: regulation time]

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "overtime":
   anchor watch, bit, day shift, dogwatch, full time, graveyard shift,
   half time, lobster trick, night shift, part time, relay, shift,
   split schedule, split shift, stint, sunrise watch, swing shift,
   time, tour, tour of duty, trick, turn, turn of work, watch,
   work shift