What is a single massive stone or rock, placed as a monument or marker known as?

A single massive stone or rock placed as a monument or marker is known as a monolith. This term specifically refers to a large stone that has been used in a monument or construction, often standing alone or as part of a larger structure. Monoliths are historically significant and can be associated with various cultural and ceremonial practices, serving as markers for territories, events, or religious sites.

The characteristics that define a monolith include its size and singularity, which distinguishes it from other types of stones or constructions like pillars or obelisks. A pillar typically refers to a column or upright structure that supports something more; a marker is a more general term that could denote anything used to mark a location, without specifying size or structure; and an obelisk is a specific type of monument with a four-sided, narrow tapering shape and usually a pyramidion on top. While all these terms relate to monuments to some extent, "monolith" is the most apt descriptor for a singular, massive stone used as a marker.