Line of longitude and Prime meridian

Line of longitude noun - An imaginary great circle on the surface of the earth passing through the north and south poles at right angles to the equator.

Prime meridian and line of longitude are semantically related. In some cases you can replace phrase "Prime meridian" with "Line of longitude", this phrases are similar.

Prime meridian noun - Meridian at zero degree longitude from which east and west are reckoned (usually the Greenwich longitude in England).

Line of longitude and prime meridian are semantically related. You can use "Line of longitude" instead a noun phrase "Prime meridian".