The Collaborative International Dictionary of English v.0.48:

Pay \Pay\, n.
   1. Satisfaction; content. --Chaucer.
      [1913 Webster]

   2. An equivalent or return for money due, goods purchased, or
      services performed; salary or wages for work or service;
      compensation; recompense; payment; hire; as, the pay of a
      clerk; the pay of a soldier.
      [1913 Webster]

            Where only merit constant pay receives. --Pope.
      [1913 Webster]

            There is neither pay nor plunder to be got.
                                                  --L'Estrange.
      [1913 Webster]

   Full pay, the whole amount of wages or salary; maximum pay;
      especially, the highest pay or allowance to civil or
      military officers of a certain rank, without deductions.


   Half pay. See under Half.

   Pay day, the day of settlement of accounts.

   Pay dirt (Mining), earth which yields a profit to the
      miner. [Western U.S.]

   Pay office, a place where payment is made.

   Pay roll,
      (a) a roll or list of persons entitled to payment, with
          the amounts due; now usually one word, payroll.
      (b) the total sum of money which is paid to all employees
          on payday.
          [1913 Webster +PJC]

The Collaborative International Dictionary of English v.0.48:

Pay dirt \Pay dirt\, Pay rock \Pay rock\, etc.
   1. (Mining) Earth, rock, etc., which yields a profit to the
      miner. [Western U. S.]
      [Webster 1913 Suppl.]

   2. a discovery, especially after an exploratory process,
      which yields a profit or sought-after benefit; -- used
      especially in the phrase "to hit pay dirt".
      [PJC]

WordNet (r) 3.0 (2006):

pay dirt
    n 1: ore that yields a substantial profit to the miner
    2: a profitable success; "the inventor worked for years before
       hitting pay dirt"