Any enclosure or exclosure including buildings, towns and cities that is overly populated by people of nigerian origin.
A "nigeria" can often be discerned by its populants unconvincing american accents, american brand clothing and (in younger circles)over use of the term "eh blud".*
*(english translation:excuse me brother)
One can often discourage the formation of a "nigeria " in an area by regularly shouting "IMMIGRATION" at random intervals
see also: visa, indefinate leave to stay, immigration.
e.g there is a growing nigeria in south east london. primarily in the areas of woolwich and peckham