stone signpost; milepost; boundary stone
A stone placed to indicate a location, distance, or boundary, such as a milestone or property marker.
山道に古い標石が立っていた。
An old stone signpost stood along the mountain path.
stone marker (surveying)
A stone used as a surveying reference point, such as a benchmark or triangulation pillar.
測量のため、山頂に標石が設置された。
A stone marker was installed on the summit for surveying.
This boundary stone marks the prefectural border.
Compound of 標 (marker, sign) and 石 (stone). The exact historical derivation is uncertain, but the word transparently means 'marker stone'.