The use of personal details of another in order to use their identity dishonestly; for example, in order to obtain money, credit etc. fraudulently.
This term originated in the USA in the 1990s. The first known record of 'identity theft' appears in The Boston Globe, May 1991:
The Social Security Administration in Boston... agrees that identity theft can be quickly stopped. 'If you can document someone else is using your Social Security number, we'll give you a new number.'
There is an earlier record of an 'identity thief', which dates from the 1960s. The Ohio newspaper The Athens Messenger, carried a story headed 'Death Reveals Identity Thief', on 7th April 1966. This reports a story of a US serviceman who stole the identity of another soldier, apparently in order to enlist fraudulently in the army:
A stranger than fiction story emerged today about a man who was killed in Vietnam while serving in the Marine Corps under the name of an acquaintance. The dead Marine has been identified as William F. Joyce, 25, of Stoughton, Mass., who was killed in action at Quan Lai while serving with the 1st Battalion of the 7th Marines.
But Joyce apparently was known to his comrades in Company C, not as Joyce, but as "Richard J. Preskenis," the name under which he enlisted three years ago. There is a Richard J. Preskenis, who went to high school in Stoughton, but who now lives in an adjoining town, Canton. Preskenis, 22, knew Joyce several years ago, but the men were not close.
Preskenis' name turned up on a casualty list issued Wednesday by the Defense Department. The notice was sent to Brockton, where the dead Marine had listed as his secondary next of kin a sister, Mrs. Paul Collins. The Marine Corps in Washington says it was she who told them that the true name of the dead Marine was Joyce.
Joyce apparently used the name of Preskenis to enlist in the Marine Corps in April, 1963, because he had had some trouble with the police, which might have made him unacceptable.
See other phrases that were coined in the USA.