A form of policing that allows no crime or anti-social behaviour to be overlooked.
As the name of a form of policing this term came into use in the USA in the 1970's. This method typically involved allocating additional law-enforcement resources to areas where some form of crime, e.g. mugging or prostitution, was endemic and then applying the strict and uncompromising letter of the law. The term was reported in The New York Times in December 1972:
"Federal officials say the calculations were based on 'assuming zero tolerance' from now on for ineligibility and overpayments."
The person most closely associated with the policy of zero tolerance on drug crime in the USA was William von Raab, who was the U.S. Commissioner of Customs during the Reagan and first Bush administrations and an architect of the so-called 'War on Drugs'.
The name, as well as the policing method, has since migrated to other countries. The term appears to have been adapted from a previous usage by the US Food and Drug Administration, where 'zero tolerance' was used to describe the amounts of pesticides that were allowable in foodstuff, like milk for example. This usage was widespread in the 1950s and 1960s. The earliest citation that I have found of that is from the Michigan newspaper The News-Palladium, October 1954:
Hootman said the zero tolerance proposed for mercury sprays might be the most troublesome to growers.
The term was probably in use in engineering circles prior to the 1950s. In October 1943, The Ogden Standard-Examiner reported a precision machine tool called a 'Zero-Tol':
"The device makes it possible to set four tools at once. After the tools are set up, they return to their exact positions without special attention from the operator."
See other phrases that were coined in the USA.