The Sicilian Mafia or Cosa Nostra (Italian: [ˈkɔːza ˈnɔstra, ˈkɔːsa -]; Sicilian: [ˈkɔːsa ˈnɔʂː(ɽ)a]; lit. 'Our Thing'), also simply referred to as the Mafia, is a criminal society and criminal organization originating on the island of Sicily and dates back to the mid-19th century. Emerging as a form of local protection and control over land and agriculture, the Mafia gradually evolved into a powerful criminal network. By the mid-20th century, it had infiltrated politics, construction, and finance, later expanding into drug trafficking, money laundering, and other crimes. At its core, the Mafia engages in protection racketeering, arbitrating disputes between criminals, and organizing and overseeing illegal agreements and transactions.

The basic group is known as a "family", "clan", or cosca. Each family claims sovereignty over a territory, usually a town, village or neighborhood (borgata) of a larger city, in which it operates its rackets. Its members call themselves "men of honour", although the public often refers to them as mafiosi. By the 20th century, wide-scale emigration from Sicily led to the formation of mafiosi style gangs in other countries, in particular in the United States, where its offshoot, the American Mafia, was created. These diaspora-based outfits replicated the traditions and methods of their Sicilian ancestors to varying extents.

The word mafia originated in Sicily. The Sicilian noun mafiusu (in Italian: mafioso) roughly translates to mean "swagger", but can also be translated as "boldness, bravado". In reference to a man, mafiusu in 19th-century Sicily was ambiguous, signifying strength, arrogant but also fearless, enterprising and proud, according to scholar Diego Gambetta. In reference to a woman, however, the feminine form "mafiusa" means a beautiful or attractive female. The Sicilian word mafie refers to the caves near Trapani and Marsala, which were often used as hiding places for refugees and criminals.

Sicily was once an Islamic emirate, and it has been challenged that a Sicilian mafia might have Arabic roots. Possible Arabic roots of the word include:

The public's association of the word with the criminal secret society was perhaps inspired by the 1863 play "I mafiusi di la Vicaria [it]" ("The Mafiosi of the Vicaria") by Giuseppe Rizzotto and Gaspare Mosca. The words mafia and mafiusi are never mentioned in the play. The drama is about a Palermo prison gang with traits similar to the Mafia: a boss, an initiation ritual, and talk of umirtà (omertà or code of silence) and "pizzu" (a codeword for extortion money). The play had great success throughout Italy. Soon after, the use of the term "mafia" began appearing in the Italian state's early reports on the group. The word was first documented in 1865 in a report by police prefect Filippo Antonio Gualterio [it].

The term mafia has become a generic term for any organized criminal network with similar structure, methods, and interests. But Giovanni Falcone, an Italian judge who was murdered by the Mafia in 1992, had objected to the conflation of the term "Mafia" with organized crime in general:

While there was a time when people were reluctant to pronounce the word "Mafia" ... nowadays people have gone so far in the opposite direction that it has become an overused term ... I am no longer willing to accept the habit of speaking of the Mafia in descriptive and all-inclusive terms that make it possible to stack up phenomena that are indeed related to the field of organised crime but that have little or nothing in common with the Mafia.

— Giovanni Falcone, 1990