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]