The Collaborative International Dictionary of English v.0.48:

Leakage \Leak"age\, n. [Cf. D. lekkage, for sense 1.]
   1. A leaking; also, the quantity that enters or issues by
      leaking.
      [1913 Webster]

   2. (Com.) An allowance of a certain rate per cent for the
      leaking of casks, or waste of liquors by leaking.
      [1913 Webster]

   3. (Elec.) A leak[3]; also; the quantity of electricity
      thus wasted.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

leakage
    n 1: the discharge of a fluid from some container; "they tried
         to stop the escape of gas from the damaged pipe"; "he had
         to clean up the leak" [syn: escape, leak, leakage,
         outflow]

Moby Thesaurus II by Grady Ward, 1.0:

78 Moby Thesaurus words for "leakage":
   ablation, access, admission, break, breakout, consumption,
   decrease, decrement, deliverance, delivery, depletion,
   depreciation, dissipation, distillation, drain, dribble, drip,
   dripping, drippings, drop, emergence, entrance, entree, entry,
   erosion, escape, escapism, evaporation, evasion, exhaustion,
   expenditure, extrication, flight, freeing, getaway, import,
   importation, importing, impoverishment, income, incoming,
   infiltration, ingoing, ingress, ingression, input, insertion,
   insinuation, intake, interpenetration, introduction, introgression,
   intrusion, issuance, issue, jailbreak, leak, leaking, liberation,
   outlet, penetration, percolation, prisonbreak, reception, release,
   rescue, riddance, seepage, setting-free, shrinkage, trickle, using,
   using up, vent, wastage, waste, wearing, wearing away



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

LEAKAGE. The waste which has taken place in liquids, by their escaping out
of the casks or vessels in which they were kept. By the act of March 2,
1799, s. 59, 1 Story's L. U. S, 625, it is provided that there be an
allowance of two per cent for leakage, on the quantity which shall appear by
the gauge to be contained in any cask of liquors, subject to duty by the
gallon and ten per cent on all beer, ale, and porter, in bottles and five
per cent on all other liquors in bottles; to be deducted, from the invoice
quantity, in lieu of breakage or it shall be lawful to compute the duties on
the actual quantity, to be ascertained by tale, at the option of the
importer, to be made at the time of entry.