In accountancy, the double-entry bookkeeping (or double-entry accounting) system is the basis of the standard system used by businesses and other organizations to record financial transactions. It was firstly described by the Italian mathematician Luca Pacioli, in its Summa de arithmetica, geometrica, proportioni et proportionalita (Venice, 1494). Its premise is that a business's (or other organization's) financial condition and results of operations are best recorded in accounts. Each account maintains a "history" of changes in monetary values about a particular aspect of the business.
This system is called double-entry because each transaction is recorded in at least two accounts.http://en.wikipedia.org/wiki/Double-entry_bookkeeping_system