tropic: either of two parallels of latitude about 23.5 degrees to the north and south of the equator representing the points farthest north and south at which the sun can shine directly overhead and constituting the boundaries of the Torrid Zone or tropics
latitude: an imaginary line around the Earth parallel to the equator
plasma: (physical chemistry) a fourth state of matter distinct from solid or liquid or gas and present in stars and fusion reactors; a gas becomes a plasma when it is heated until the atoms lose all their electrons, leaving a highly electrified collection of nuclei and free electrons