What is the definition of a Meridian?

A Meridian is defined as a semi great circle that runs north to south from pole to pole. This definition is rooted in the geographic coordinate system, where meridians are crucial for establishing longitude. They converge at the poles and are perpendicular to the lines of latitude, helping to create a grid for navigation and mapping the Earth's surface.

In addition, understanding the concept of meridians is essential in aviation and navigation, as pilots utilize them to determine positions relative to the Earth's coordinate system. Knowing that meridians are oriented north-south helps pilots and navigators accurately plot their courses and maintain awareness of their geographic location during flight.

The other options refer to concepts that either relate to geographical navigation tools or different types of circles on Earth. A semi great circle running east to west refers to lines of latitude, while navigation tools and reference points for latitude involve different elements of geographic or navigational concepts, not directly defining a meridian itself.