In many countries, Kilometre Zero (also written km 0) or similar terms in other languages, is a particular location (often in the nation's capital city), from which distances are traditionally measured. A similar notion also exists for individual roads (that is, all locations on the road have a number, depending on their distance from that location), and for individual cities (often the city's central post office is used for this).

The most famous such marker of which any part survives from ancient times is the Milliarium Aureum ("Golden Milestone") of the Roman Empire, believed to be the literal origin for the maxim that "all roads lead to Rome".

Argentina marks Kilometre Zero with a monolith in Plaza Congreso in Buenos Aires. The work of the brothers Máximo and José Fioravanti, the structure was placed on the north side of Plaza Lorea on October 2, 1935; it was moved to its present location on May 18, 1944. An image of Our Lady of Luján (honored on the monolith as "the patron saint of the national road network") appears on the monolith's north face, a relief map of Argentina is on the south face, plaques in honor of José de San Martín are west, and on its eastern side, the date of the decree and the name of the relevant authorities.

The kilometre (American spelling: kilometer; SI symbol: km) is a unit of length in the metric system, equal to one thousand metres (kilo- being the SI prefix for 1000). It is now the measurement unit used officially for expressing distances between geographical places on land in most of the world; notable exceptions are the United States and the United Kingdom where the statute mile is the official unit used.

k (pronounced kay) is occasionally used in some English-speaking countries as an alternative for the word kilometre in everyday writing and speech. A slang term for the kilometre in the U.S. military is klick.

There are two common pronunciations for the word:

The former pronunciation follows the general pattern in English whereby SI units of measurement are pronounced with the stress on the first syllable, and the pronunciation of the actual base unit does not change irresepective of the prefix. It is generally preferred by the British Broadcasting Corporation (BBC) and the Australian Broadcasting Corporation (ABC).