Question

Explain why the lines of longitude are called meridians of longitude

Answer 1

A meridian (or line of longitude) is the half of an imaginary great circle on the Earth's surface, terminated by the North Pole and the South Pole, connecting points of equal longitude, as measured in angular degrees east or west of the Prime Meridian.

Answer 2

longitude are known as meridians because in geographical sense, meridians are great circles which are not parallel to each other but intersect each other at North and South poles.

same stands true for longitudes. All longitudes are great circles which meat at the poles.