NOUN (3)

1. formerly the restricted quarter of many European cities in which Jews were required to live;
- Example: "the Warsaw ghetto"

2. any segregated mode of living or working that results from bias or stereotyping;
- Example: "the relative security of the gay ghetto"
- Example: "no escape from the ghetto of the typing pool"

3. a poor densely populated city district occupied by a minority ethnic group linked together by economic hardship and social restrictions;

The Collaborative International Dictionary of English v.0.48:

Ghetto \Ghet"to\, n. [It.]
   A quarter of a city where Jews live in greatest numbers.
   [Webster 1913 Suppl.]

         I went to the Ghetto, where the Jews dwell. --Evelyn.
   [Webster 1913 Suppl.]

   2. By extension: Any section of a town inhabited
      predominantly by members of a specific ethnic, national or
      racial group, such segregation usually arising from social
      or economic pressure. The term is commonly applied to
      areas in cities having a high concentration of low-income
      African-Americans.
      [PJC]

   3. [fig.] Any isolated group of people.
      [PJC]

   4. [fig.] Any group isolated by external pressures, with an
      implication of inferiority.
      [PJC]

WordNet (r) 3.0 (2006):

ghetto
    n 1: formerly the restricted quarter of many European cities in
         which Jews were required to live; "the Warsaw ghetto"
    2: any segregated mode of living or working that results from
       bias or stereotyping; "the relative security of the gay
       ghetto"; "no escape from the ghetto of the typing pool"
    3: a poor densely populated city district occupied by a minority
       ethnic group linked together by economic hardship and social
       restrictions

Moby Thesaurus II by Grady Ward, 1.0:

34 Moby Thesaurus words for "ghetto":
   Bowery, Chinatown, East End, East Side, Little Hungary,
   Little Italy, West End, West Side, barrio, black ghetto,
   blighted area, business district, central city, city center, core,
   downtown, greenbelt, inner city, midtown, outskirts,
   red-light district, residential district, run-down neighborhood,
   shopping center, skid road, skid row, slum, slums, suburbia,
   suburbs, tenderloin, tenement district, uptown, urban blight