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