In law and government, appropriation (from Latin appropriare, "to make one's own", later "to set aside") is the act of setting apart something for its application to a particular usage, to the exclusion of all other uses.
Ecclesiastical law 
In ecclesiastical law, appropriation is the perpetual annexation of an ecclesiastical benefice to the use of some spiritual corporation, either aggregate or sole. In the Middle Ages in England the custom grew up of the monasteries reserving to their own use the greater part of the tithes of their appropriated benefices, leaving only a small portion to their vicars in the parishes. On the dissolution of the monasteries the rights to collect "great tithes" were often sold off, along with former monastic lands, to laymen; whose successors, known as "lay impropriators" or "lay rectors," still hold them, the system being known as impropriation.
Law of debtor and creditor 
In the law of debtor and creditor, appropriation of payments is the application of a particular payment for the purpose of paying a particular debt. When a creditor has two debts due to him from the same debtor on distinct accounts, the general law as to the appropriation of payments made by the debtor is that the debtor is entitled to apply the payments to such account as he thinks fit; solvitur in modum solventis. In default of appropriation by the debtor the creditor is entitled to determine the application of the sums paid, and may appropriate them even to the discharge of debts barred by the Statute of Limitations. In default of appropriation by either debtor or creditor, the law implies an appropriation of the earlier payments to the earlier debts.
In constitutional law, appropriation is the assignment of money for a special purpose.
In the United Kingdom an appropriation act appropriating various sums to government departments and quangos, as set out by the government.
The United States Constitution (art. I. § 9) says: "No money shall be drawn from the treasury, but in consequence of appropriations made by law." The most important statutes of this sort are the 13 annual appropriation bills, by which definite sums are reserved for specified objects. Bills for appropriating money must originate in the House of Representatives, but may be amended in the Senate.
Criminal Law 
In England and Wales, the Theft Act 1968 s 3(1) defines appropriation as "Any assumption by a person of the rights of an owner"
A contract authority is a form of budget authority that permits obligations to be made in advance of appropriations.