The certificate issued by the DVLA after a vehicle passes inspection, confirming it is fit to be on the road.
Private vehicles are inspected less often than commercial ones, and the sticker goes on the windscreen. Police checkpoints ask for it alongside your licence and your insurance.
The inspection covers brakes, lights, tyres, steering, emissions, and the chassis and engine numbers. That last part matters: it is also how a stolen or misdescribed vehicle gets caught.