Noun - any gadget or piece of software used to switch off environmental controls and allow pollution to pump into the air.
This is a marvellously laconic phrase, brought to the world by the US Clean Air Act in 1970. Since then, many companies have been caught indulging in the trickery of a defeat device but it took this year's Volkswagen scandal - breathtaking in its scope - to bring it to wider attention.
It also contains an irony. The software surreptitiously installed by Volkswagen engineers on about 11 million diesel cars was intended to defeat regulatory limits on nitrogen oxide emissions. Instead, the device ended up defeating Volkswagen, exposing it as a lawbreaker and a firm that relied on deception in its quest to become the biggest car company.
This capped a year in which routine deceptions were exposed in other industries - notably banking. Tom Hayes, the UBS trader jailed for 11 years for organising the rigging of Libor indexes, told detectives that he had operated in a system in which such shenanigans "were commonplace". Any regulation presented an opportunity to bend it.
"Defeat device" sounds innocuous compared with "fraud". Yet that is what it really meant. Millions of drivers had a stamp of approval that their vehicles were clean when some emitted up to 40 times the registered volume of smog particles. The deception put into question the future of diesel technology.
How does rule-bending turn into rule-breaking? Slowly, steadily but often inevitably. Companies that should compete to produce better products instead vie to defeat regulations.
A defeat device is a box of tricks for a niche audience of engineers and regulators. When the public gets to hear about it, it does not seem so clever.
THE FINANCIAL TIMES