Gentrification is the process whereby the character of a neighborhood changes through the influx of more affluent residents (the "gentry") and investment. There is no agreed-upon definition of gentrification. In public discourse, it has been used to describe a wide array of phenomena, sometimes in a pejorative connotation.

Gentrification is a common and controversial topic in urban politics and planning. Gentrification often increases the economic value of a neighborhood, but can be controversial due to changing demographic composition and potential displacement of incumbent residents. Gentrification is more likely when there is an undersupply of housing and rising home values in a metropolitan area.

The gentrification process is typically the result of increasing attraction to an area by people with higher incomes spilling over from neighboring cities, towns, or neighborhoods. Further steps are increased investments in a community and the related infrastructure by real estate development businesses, local government, or community activists and resulting economic development, increased attraction of business, and lower crime rates.

Historians say that gentrification took place in ancient Rome and in Roman Britain, where large villas were replacing small shops by the 3rd century, AD. The word gentrification derives from gentry—which comes from the Old French word genterise, "of gentle birth" (14th century) and "people of gentle birth" (16th century). In England, landed gentry denoted the social class, consisting of gentlemen (and gentlewomen, as they were known at that time).

A more direct derivational base of gentrification is the 19th-century neologism 'gentrify,' a verb coined by Samuel Laing (1780–1868). This term reflected shifting societal attitudes—specifically, the idea that one could attain upper-class status through conduct rather than birth—while also introducing undertones of conspicuous consumption and pretentiousness.

British sociologist Ruth Glass was first to use "gentrification" in its current sense. She used it in 1964 to describe the influx of middle-class people displacing lower-class worker residents in urban neighborhoods; her example was London, and its working-class districts such as Islington:

One by one, many of the working class neighbourhoods of London have been invaded by the middle-classes—upper and lower. Shabby, modest mews and cottages—two rooms up and two down—have been taken over, when their leases have expired, and have become elegant, expensive residences ... Once this process of 'gentrification' starts in a district it goes on rapidly, until all or most of the original working-class occupiers are displaced and the whole social character of the district is changed.

In the US, the Centers for Disease Control and Prevention report Health Effects of Gentrification defines the real estate concept of gentrification as "the transformation of neighborhoods from low value to high value." A real estate encyclopedia defines gentrification as "the process by which central urban neighborhoods that have undergone disinvestments and economic decline experience a reversal, reinvestment, and the in-migration of a well-off middle- and upper-middle-class population."