The Collaborative International Dictionary of English v.0.48:

Jackpot \Jackpot\, n.
   1. Same as jack pot. See under jack.
      [PJC]

   2. Any larger-than-usual gambling prize formed by the
      accumulation of unwon bets.
      [PJC]

   3. The highest gambling prize awarded in a gambling game in
      which smaller prizes are also awarded, especially such a
      prize on a slot machine.
      [PJC]

   4. An unusually large success in an enterprise, either
      unexpected or unpredictable, esp. one providing a great
      financial benefit.
      [PJC]

   hit the jackpot to receive an unexpectedly large (or the
      largest possible) benefit from an enterprise; as, after
      prospecting for years, he finally hit the jackpot when he
      discovered a silver lode.
      [PJC]

WordNet (r) 3.0 (2006):

hit the jackpot
    v 1: succeed by luck; "I lucked out and found the last parking
         spot in the lot" [syn: luck out, hit the jackpot]