Kilometre zero: a physical marker designating the origin point of a country's road-distance measurements. Prime meridian: the meridian defined as 0° longitude, today the IERS Reference Meridian near Greenwich. Vertical datum: a reference surface, usually tied to tide-gauge records, from which elevations are measured. Geoid: the equipotential surface of Earth's gravity field that best approximates global mean sea level.
Absolute zero: 0 K or −273.15 °C, the lower limit of temperature, reachable only as a limit. Epoch: the instant defined as zero in a time-counting system, such as 1 January 1970 for Unix time. Placeholder zero: a symbol marking an empty position in positional notation, the historical forerunner of zero as a number.