Wiktionary
n. A party thrown in a cordened-off street or road.
Wikipedia
A street party can mean any type of social event taking place on a road.
In the UK, these have historically been held to commemorate momentous events, such as VE Day or the Queen's jubilees, with bunting dressing the street, and children playing in the street. An estimated 10 million people took part in street parties in 1977 for the Queen's Silver Jubilee.
The British tradition seems to have begun after World War I as residents' own "peace teas" to celebrate the signing of the Treaty of Versailles in 1919.
The tradition was boosted for the wedding of Prince William and Kate Middleton in April 2011 with about 1 million people joining in street parties. For the Queen's Diamond Jubilee in June 2012 about 2 million took part.
Now street parties are held annually and at any time for residents to meet their neighbours in a traffic-free street in a private street party. Some 'street parties' are public events taking many forms. In the USA some are called a 'block party'.
As a form of activism street parties are festive and/or artistic efforts to reclaim roadways as public space by large groups of people. They were made known in Western Europe and North America by the actions of Reclaim the Streets, a widespread "dis-organization" dedicated to reclaiming public space from automobiles and consumerism. In a somewhat different context, Poland's Orange Alternative staged festive protests to break the Communist government's monopoly on public life.
Street Party was a music video program on MTV that aired Friday nights between the years 1989 and 1992. A companion show to Club MTV, Street Party aired dance videos. Unlike other genre shows like Headbangers Ball and 120 Minutes, Street Party was more of a video block that's length changed each week from 2 to 4 to even once 24 hours. VJ segments are reported to have been taped on the Club MTV set. In 1992, MTV lost interest in dance music and cancelled both Club MTV and Street Party, as well as removing dance videos from general rotation both on MTV and VH1.