What's the difference between meridians and longitude?

The parallels of latitude are of unequal length, whereas the meridians of longitude are of equal length. Latitude and Longitude (Meridians and Parallels) The Equator is an imaginary circle equidistant from the poles of the Earth. Circles parallel to the Equator (lines running east and west) are parallels of latitude. They are used to measure degrees of latitude north or south of the Equator

The parallels of latitude are of unequal length, whereas the meridians of longitude are of equal length.