The Collaborative International Dictionary of English v.0.48:

Usance \Us"ance\, n. [F. See Use, v. t.]
   [1913 Webster]
   1. Use; usage; employment. [Obs.] --Spenser.
      [1913 Webster]

   2. Custom; practice; usage. [Obs.] --Gower. Chaucer.
      [1913 Webster]

   3. Interest paid for money; usury. [Obs.] --Shak.
      [1913 Webster]

   4. (Com.) The time, fixed variously by the usage between
      different countries, when a bill of exchange is payable;
      as, a bill drawn on London at one usance, or at double
      usance.
      [1913 Webster]

WordNet (r) 3.0 (2006):

usance
    n 1: the period of time permitted by commercial usage for the
         payment of a bill of exchange (especially a foreign bill of
         exchange)
    2: (economics) the utilization of economic goods to satisfy
       needs or in manufacturing; "the consumption of energy has
       increased steadily" [syn: consumption, economic
       consumption, usance, use, use of goods and services]
    3: accepted or habitual practice [syn: custom, usage,
       usance]

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

USANCE, commercial law. The term usance comes from usage, and signifies the
time which by usage or custom is allowed in certain countries, for the
payment of a bill of exchange. Poth. Contr. du Change, n. 15.
     2. The time of one, two or three mouths after the date of the bill,
according to the custom of the places between which the exchanges run.
     3. Double or treble is double or treble the usual time, and half usance
is half the time. Where it is necessary to divide a month upon a half
usance, which is the case when the usance is for one month or three, the
division, notwithstanding the difference in the length of the months,
contains fifteen days.