Identity theft happens when someone uses another person’s personal identifying information, such as their name, Social Security number, or credit card number, without permission to commit fraud or other crimes.
Thieves typically use the stolen details to open credit accounts, make purchases, file fraudulent tax refunds, or obtain other benefits, and the victim may be left dealing with debts or records created in their name. The term dates to 1964 and is legally defined in both the U.S. and the U.K.
Source: Identity theft, Wikipedia