Overview
A Negroni is an Italian cocktail, made of one part gin, one part vermouth rosso (red, semi-sweet) and one part Campari, garnished with orange peel. It is considered an apéritif.
A traditionally made Negroni is stirred, not shaken; it is built over ice in an old-fashioned or rocks glass and garnished with a slice of orange. Outside of Italy, orange peel is often used in place of an orange slice.
History
The drink's origins are not known with certainty. The most widely reported account is that it was first mixed in Florence, Italy, in 1919, at Caffè Casoni, on Via de' Tornabuoni. The commonly-held origin story is that it was concocted by a member of the Negroni family asking the bartender to strengthen the Americano by adding gin, rather than the normal soda water. The bartender also added an orange garnish rather than the typical lemon garnish of the Americano to signify that it was a different drink.
From Wikipedia (CC BY-SA 4.0).