A milestone is a numbered marker placed on a route such as a road, railway line, canal or boundary. They can indicate the distance to towns, cities, and other places or landmarks like mileage signs; or they can give their position on the route relative to some datum location (a zero milepost). On roads they are typically located at the side or in a median or central reservation. They are alternatively known as mile markers (sometimes abbreviated MMs), mileposts or mile posts (sometimes abbreviated MPs). A "kilometric point" is a term used in metricated areas, where distances are commonly measured in kilometres instead of miles. "Distance marker" is a generic unit-agnostic term.
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Distance marker in the context of Kilometre zero
Kilometre zero (or km 0), also known as zero mile marker or zero milepost, is a particular location (usually in the nation's capital city) from which traveled distances are traditionally measured, as in distance markers. Historically, they were markers where drivers could set their odometers to follow the directions in early roaming guidebooks.
One such marker is the Milliarium Aureum ("Golden Milestone") of the Roman Empire, believed to be the origin for the maxim "All roads lead to Rome".
View the full Wikipedia page for Kilometre zero