The Collaborative International Dictionary of English v.0.48:

Quantum \Quan"tum\, n.; pl. Quanta. [L., neuter of quantus how
   great, how much. See Quantity,]
   1. Quantity; amount. "Without authenticating . . . the
      quantum of the charges." --Burke.
      [1913 Webster]

   2. (Math.) A definite portion of a manifoldness, limited by a
      mark or by a boundary. --W. K. Clifford.
      [1913 Webster]

   Quantum meruit[L., as much as he merited] (Law), a count in
      an action grounded on a promise that the defendant would
      pay to the plaintiff for his service as much as he should
      deserve.

   Quantum sufficit, or Quantum suff. [L., as much suffices]
      (Med.), a sufficient quantity; -- abbreviated q. s. in
      pharmacy.

   Quantum valebat[L., as much at it was worth] (Law), a count
      in an action to recover of the defendant, for goods sold,
      as much as they were worth. --Blackstone.
      [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

QUANTUM VALEBAT, pleading. As much as it was worth. When goods are sold,
without specifying any price, the law implies a promise from the buyer to
the seller that he will pay him for them as much as they were worth.
     2. The plaintiff may, in such case, suggest in this declaration that
the defendant promised to pay him as much as the said goods were worth, and
then aver that they were worth so much, which the defendant has refused to
pay. Vide the authorities cited under the article Quantum meruit.