The Collaborative International Dictionary of English v.0.48:

Demurrage \De*mur"rage\, n. [Cf. OF. demorage delay. See
   Demur.] (Law)
   (a) The detention of a vessel by the freighter beyond the
       time allowed in her charter party for loading, unloading,
       or sailing.
   (b) The allowance made to the master or owner of the ship for
       such delay or detention.
       [1913 Webster]

             The claim for demurrage ceases as soon as the ship
             is cleared out and ready for sailing. --M`Culloch.
       [1913 Webster]

   Note: The term is also applied to similar delays and
         allowances in land carriage, by wagons, railroads, etc.
         [1913 Webster]

WordNet (r) 3.0 (2006):

demurrage
    n 1: a charge required as compensation for the delay of a ship
         or freight car or other cargo beyond its scheduled time of
         departure
    2: detention of a ship or freight car or other cargo beyond its
       scheduled time of departure

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

DEMURRAGE, mar. law. The freighter of a ship is bound not to detain it,
beyond the stipulated or usual time, to load, or to deliver the cargo, or to
sail. The extra days beyond the lay days (being the days allowed to load and
unload the cargo), are called the days of demurrage; and that term is
likewise applied to the payment for such delay, and it may become due,
either by the ship's detention, for the purpose of loading or unloading the
cargo, either before, or during, or after the voyage, or in waiting for
convoy. 3 Kent, Com. 159; 2 Marsh, 721; Abbott on Ship. 192 5 Com. Dig. 94,
n., 505; 4 Taunt. 54, 55; 3 Chit. Com. Law, 426; Harr. Dig. Ship and
Shipping, VII.