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]