What is the term for the point, line, or surface from which elevations are measured?

The term for the point, line, or surface from which elevations are measured is referred to as "datum." This concept is crucial in various fields such as surveying, engineering, and construction. A datum serves as a consistent reference point to determine the elevation of other points relative to it. For instance, in surveying, a datum might be a known sea level or another established reference to ensure that measurements are accurate and consistent across different locations.

By using a datum, surveyors can create a reliable framework for mapping terrain or planning construction projects, ensuring that all elevations are calculated from the same baseline. This standardization is essential for clarity and precision in topographic maps and engineering designs.

Other options like a reference point and benchmark serve related purposes but do not specifically denote the established imaginary plane or level from which measurements are derived. Hence, "datum" is the precise terminology that encapsulates this definition within the context of elevation measurement.