What is the name of the imaginary line that runs east and west and separates north and south?

The imaginary line that runs east and west to separate north and south is known as the Base Line. In the context of land surveying and mapping, base lines are used as reference points from which land is described and measured. They work in conjunction with the principal meridian, which runs north and south. The base line is essential for creating a coordinate system for navigating and dividing land into parcels, especially in the United States and other regions that use the Public Land Survey System (PLSS).

Understanding the function of a base line in relation to land division and surveying can clarify why this term is specifically applied to the east-west line used for such purposes. While latitude lines are also drawn east and west, they denote degrees of latitude rather than serving as official surveying references. Thus, the terminology best suited for this context is indeed the Base Line.