Euro 6
Euro 6 is the European exhaust-emissions standard that has applied to all new cars since September 2015, slashing permitted nitrogen oxides (NOx) from diesels in particular. It matters enormously for used buyers: Euro 6 diesels are exempt from London’s ULEZ charge and most clean air zones, while older diesels pay daily. Petrol cars only needed Euro 4 for the same exemption — a lower bar most 2006+ cars clear.
Example
A September 2015 diesel is (just) Euro 6 and ULEZ-exempt; an August 2015 one may not be — check before buying.