What is Barter?

Legal Definition
Barter is a system of exchange where goods or services are directly exchanged for other goods or services without using a medium of exchange, such as money. It is distinguishable from gift economies in many ways; one of them is that the reciprocal exchange is immediate and not delayed in time. It is usually bilateral, but may be multilateral (i.e., mediated through barter organizations) and, in most developed countries, usually only exists parallel to monetary systems to a very limited extent. Barter, as a replacement for money as the method of exchange, is used in times of monetary crisis, such as when the currency may be either unstable (e.g., hyperinflation or deflationary spiral) or simply unavailable for conducting commerce.

Economists since Adam Smith, looking at non-specific archaic societies as examples, have used the inefficiency of barter to explain the emergence of money, the economy, and hence the discipline of economics itself. However, ethnographic studies have shown no present or past society has used barter without any other medium of exchange or measurement, nor have anthropologists found evidence that money emerged from barter, instead finding that gift-giving (credit extended on a personal basis with an inter-personal balance maintained over the long term) was the most usual means of exchange of goods and services.

Since the 1830s, barter in some western market economies has been aided by exchanges that use alternative currencies based on the labour theory of value, and are designed to prevent profit taking by intermediators. Examples include the Owenite socialists, the Cincinnati Time store, and more recently Ithaca HOURS (Time banking) and the LETS system.
-- Wikipedia
Legal Definition
A contract by which the parties exchange goods for goods. To complete the contract the goods must be delivered, for without a delivery, the right of property is not changed.

2. This contract differs from a sale in this, that barter is always of goods for goods, whereas a sale is an exchange of goods for money. In the former there never is a price fixed, in the latter a price is indispensable. All the differences which may be pointed out betwen these two contracts, are comprised in this; it is its necessary consequence. When the contract is an exchange of goods on one side, and on the other side the consideration is partly goods and partly money, the contract is not a barter, but a sale. See Price; Sale.

3. If an insurance be made upon returns from a country where trade is carried on by barter, the valuation of the goods in return shall be made on the cost of those given in barter, adding all charges. Wesk. on Ins. 42. See 3 Camp. 351 Cowp. 818; 1 Dougl. 24, n.; 1 N. R. 151 Tropl. de l'Echange.
-- Bouviers Law Dictionary
Legal Definition
A contract by which parties exchange goods or commodities for other goods. It differs from sale, in this: that in the latter transaction goods or property are always exchanged for money. Guerreiro v. Peile, 3 Barn. & Aid. 617; Cooper v. State, 37 Ark. 418; Meyer v. Rousseau, 47 Aria 460, 2 S. W. 112. This term is not applied to contracts con-corning land, but to such only as relate to goods and chattels. Barter is a contract by which the parties exchange goods. Speigle v. Meredith, 4 Biss. 123, Fed. Can. No. 13,227.
-- Black's Law Dictionary
Legal Definition
An exchange of goods.
-- Ballentine's Law Dictionary