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.
2. (Math.) A definite portion of a manifoldness, limited by a
mark or by a boundary. --W. K. Clifford.
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
Quantum sufficit, or Quantum suff. [L., as much suffices]
(Med.), a sufficient quantity; -- abbreviated q. s. in
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.