Final answer:
The imaginary line running vertically down the body, dividing it into left and right parts, is called the midline, which corresponds to the anatomical midsagittal plane.
Step-by-step explanation:
The imaginary line running vertically down the body, dividing it into left and right parts, is known as the midline. In anatomical terms, a midsagittal plane is the specific plane that divides the body into equal left and right halves. This is different from geographic terms of orientation such as the equator and prime meridian, which are used to organize locations on Earth. The equator is the largest circle of latitude, dividing the Earth into the Northern and Southern Hemispheres, and has no attached letter indicating direction, unlike other latitudinal lines. The prime meridian, set at 0 degrees longitude, divides the Earth into the Eastern and Western Hemispheres. When we discuss the human body, it is the midline and midsagittal plane that are most relevant for anatomical division.