A milestone or other object placed to indicate the distance of a mile from a starting point or from another mark; (also) a distance of a mile. Also…

A stone or mark set up by the ancient Romans as a point of departure in measuring distances of a thousand paces; a milestone. Now rare.

A stone or (usually short) pillar set up beside a road indicating the distance in miles from that point to a particular place.

a marker indicating the route and often distance to another point (frequently one of a series, regularly spaced); spec. a point or marker one mile…

= milepost n.