A ghetto is a part of a city in which members of a minority group are concentrated, especially as a result of political, social, legal, religious, environmental or economic pressure. Ghettos are often known for being more impoverished than other areas of the city. Versions of such restricted areas have been found across the world, each with their own names, classifications, and groupings of people.

The term was originally used for the Venetian Ghetto in Venice, Italy, as early as 1516, to describe the part of the city where Jewish people were restricted to live and thus segregated from other people. However, other early societies may have formed their own versions of the same structure; words resembling ghetto in meaning appear in Hebrew, Yiddish, Italian, Germanic, Polish, Corsican, Old French, and Latin. During the Holocaust, more than 1,000 Nazi ghettos were established to hold the Jewish populations of Europe, with the goal of exploiting and killing European Jews as part of the Final Solution of Nazi Germany.

The term ghetto acquired deep cultural meaning in the United States, especially in the context of segregation and civil rights. It has been widely used in the country since the 20th century to refer to poor neighborhoods of largely minority populations. It is also used in some European countries, such as Romania and Slovakia, to refer to poor neighborhoods largely inhabited by Romani people. The term slum is usually used to refer to areas in developing countries that suffer from absolute poverty, while the term ghetto is used to refer to areas of developed countries that suffer from relative poverty.

The word ghetto originates from the Venetian ghetto, the Jewish quarter in Venice's Cannaregio district where Jews were legally confined following a 1516 decree. In the 16th century, Italian Jews, including those in Venice, commonly used the unrelated Hebrew term ḥāṣēr ('courtyard') to refer to a ghetto. By 1855, the term ghetto had been extended to refer to "any area occupied predominantly by a particular social or ethnic group, especially a densely populated urban area which is subject to social and economic pressures, tending to restrict its demographic profile."

The etymology of the Italian ghetto has long been debated among linguists, with no single theory achieving universal acceptance. Although often cited, the idea that it derives from the Hebrew gēṭ ('letter of divorce'; because the ghetto separated Jews from the rest of the population) is considered a folk etymology. Similarly, the Italian variant ghet, found in some Jewish notarial documents from the late 16th and early 17th centuries, seems to be a folk-etymological modification of ghetto, influenced by the Hebrew gēṭ.

Another commonly held hypothesis, mentioned by the Oxford English Dictionary, proposes that the term comes from an unattested Italian *gheto ('foundry'; cf. Italian getto 'the process of founding or casting metal'; 14th cent.), from post-classical Latin iectus or iactus (attested in a 1295 Venetian source referring to the locality), which could be compared to post-classical Latin ghetus or gettus (attested from 1306 in Venetian sources referring to the locality). However, linguist Anatoly Liberman argues that this explanation fails to account for the problematic phonetic change from Latin i- to Italian g- ~ gh-, and that there is no certainty that getto ever meant 'foundry' in the Venetian dialect. Alternatively, Liberman has suggested that in some Romance-speaking regions a slang borrowing from the Germanic gata ('street' or 'narrow street') may have existed in various forms, which eventually evolved into the Italian ghetto. Originally, the term may have carried a derogatory sense, referring to the impoverished quarters of exiled Venetian Jews. Over time, folk etymology further shaped its meaning, associating it with ideas like cannon foundries and separation.

Other suggestions, such as that the word is a shortening of Egitto ('Egypt') or borghetto ('small settlement'), or that it is related to the Old French guect ('guard'), are rejected by linguists as speculative and unconvincing. Additional proposals derive the term from ghectus (understood as the Latinized form of Yiddish gehektes 'enclosed') or from the Latin neuter Giudaicetum ('Jewish'), but they too lack sufficient phonetic support.

The character of ghettos has varied through times. The term was used for an area known as the Jewish quarter, which meant the area of a city traditionally inhabited by Jews in the diaspora. Jewish quarters, like the Jewish ghettos in Europe, were often the outgrowths of segregated ghettos instituted by the surrounding authorities. A Yiddish term for a Jewish quarter or neighborhood is Di yidishe gas (Yiddish: די ייִדישע גאַס), or 'The Jewish street'. Many European and Middle Eastern cities once had a historical Jewish quarter.^{[citation needed]} Among the oldest and most noted are the so-called calls in Catalonia, specially those of Barcelona (Call jueu) and Girona.