What is Demurrage?

Legal Definition
The term "demurrage" /dɪˈmɜːrɪ/ from Old French demeurage, from demeurer - to linger, tarry - originated in vessel chartering and referred to the period when the charterer remained in possession of the vessel after the period normally allowed to load and unload cargo (laytime). By extension, demurrage refers to the charges that the charterer pays to the shipowner for its extra use of the vessel. Officially, demurrage is a form of liquidated damages for breaching the laytime as a it is stated in the governing contract (the charter party). The demurrage sometimes causes a loss to the seller as it increases cost of the total freight.

The reverse of demurrage is despatch. If the charterer requires the use of the vessel for less time than the laytime allowed, the charter party may require the shipowner to pay despatch for the time saved.

The term demurrage has been incorrectly extended to use in the hire or rental of assets other than ships. Loss of use resulting in additional hire is not demurrage.
-- Wikipedia
Legal Definition
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.
-- Bouviers Law Dictionary
Legal Definition
In maritime law. The sum which is fixed by the contract of carriage or which is allowed, as remuneration to the owner of a ship for the detention of his vessel beyond the number of days allowed by the charter-party for loading and unloading or for sailing. Also the detention of the vessel by the freighter beyond such time. See 3 Kent, Comm. 203 ; 2 Steph. Comm. 185. The Apollon, 9 Wheat. 378, 6 In Ed. Ill; Fisher v. Abeel, 44 How. Prae. (N. Y.) 440; Wordin v. Bemis, 32 Conn. 273, 85 Am. Dee, 255; Cross v. Beard, 26 N. Y. 85; The J. E. Owen (D. Ct) 54 Fed. 185; Falkenburg v. Clark, 11 R. I. 283.

Demurrage is only an extended freight or reward to the vessel, in compensation for the earnings she is improperly caused to lose. Every improper detention of a vessel may be considered a demurrage, and compensation under that name be obtained for it. Donaldson v. McDowell, Holmes, 290, Fed Cas. No. 3,985. Demurrage is the allowance or compensation due to the master or owners of a ship, by the freighter, for the time the vessel may have been detained beyond the time specified or implied in the contract of affreightment or the charter-party. Bell.
-- Black's Law Dictionary
Legal Definition
Charges for detention of a ship or a freight-car. See 30 Am. St. Rep. 634, note.
-- Ballentine's Law Dictionary