20 mph zone

A 20 mph zone includes design features aimed at keeping vehicle speeds below 20 mph, such as humps, speed tables, rough surfaces, tight geometry, road narrowings, and so on.

The aim is for the zone to be self-enforcing, through traffic calming. No repeaters are necessary. Direction 16 of the 2002 TSRGD states that 

The sign shown in diagram 674 [20mph zone] may only be placed on a road if no point on any road (not being a cul-de-sac less than 80 metres long), to which the speed limit indicated by the sign applies, is situated more than 50 metres from a traffic calming feature. 

That is - 20 mph zones can only be applied with traffic calming features at regular intervals of 100 metres or less. These should ensure that speeds naturally fall to around 20 mph. 

Not to be confused with 20mph limit, which is simply a speed limit, with or without the self-enforcing design features described here.