Question

What is the fictional line that travels around the earth between the north and the south poles?

Answer 1

It must be longitude.... I guess.
If not then it must be the Earth's axis.

Answer 2

East-west lines on the globe are Latitude. The line at zero degrees longitude, through Greenwich, England, is the Prime Meridian. If you're talking about a line directly through the poles, through the center of the Earth, that is called its Axis.