economically - any neighborhood where there has not been city or private business investment in line with other residential areas of the city

architecturally - any neighborhood within city limits with predominantly unfinished houses, unpaved roads, and very narrow roads.  

as of the 1920's the word favela stared to be used to refer to any precarious housing settlement built by people who did not own the land they occupied.

in the 1950's, the word 'favela' became an official term extended to the use by the brazilian census.