Understeer, the experts will tell you, is the tendency for a car’s front wheels to slip outwards under increasing cornering loads.
The 'slip angle' of the front tyres increases as you go faster and faster, until the tyre breaks away entirely unless it’s tamed by an ESP system.
It’s a common condition, widely demonstrable in today’s front-drive cars, whose front wheels are well laden, even in a static condition, by the mass of an engine above.
Increase that load with energetic cornering and you fairly readily reach a point where the tyre won’t go precisely where it’s pointed. Let’s call this laden understeer.
But there's also a wholly rarer form of understeer, so hard to find today that I confess I’d all but forgotten about its existence until the new Renault Twingo came along. It’s unladen understeer – a common condition once upon a time in old-timers such as the rear-engined VW Beetle, Renault R8 and R10 (which would often abruptly change to oversteer and send you backwards through the hedge) and mid-engined models such as the Lancia Montecarlo and Fiat X1/9.
Last time it came to notice, I think, was in early examples of the Smart Roadster and the Lotus Elise.