A free copy of a new publication which is sent to the copyright office, or to designated libraries, by the author or publisher under the terms of the national copyright law. In the United States, deposit copies are sent to the Library of Congress in Washington, D.C.
Some references for the history of British copyright deposit:
Feather, John. "Publishers and Politicians: The Remaking of the Law of Copyright in Britain 1775-1842. Part I: Legal Deposit and the Battle of the Library Tax." Publishing History 24 (1988): 49-76.
Harris, P. (Philip) R. The History of the British Museum Library, 1753-1973, 833. London: British Library, 1998. ISBN - 0 7123 4562 0. There is a section on copyright deposit.
Oates, J. C. T. (John Claud Trewinard). "Cambridge and the Copyright Act of Queen Anne (1710-1814)." In Quick Springs of Sense Studies in the Eighteenth Century, edited by Larry S. Champion, 61-73. Athens GA: University of Georgia Press, 1974. Published from Oates' typescript of 1972, "Copyright Deposit at Cambridge in the Eighteenth Century".
———. "The Deposit of Books at Cambridge under the Licensing Acts, 1662-70, 1685-95." Transactions of the Cambridge Bibliographical Society 2, no. 4 (1957): 290-304. Partridge, R. C. Barrington. The History of the Legal Deposit of Books. London and Birmingham: The Library Association, 1938.
Willison, Ian. "Legal Deposit: A Provisional Perspective." Publishing History 45 (1999): 5-34.