The part of the earth's outer atmosphere where free electrons, arising from ionization, are normally present in quantities sufficient to modify the propagation characteristics of radio waves traversing it.
Note: For purposes of reference, the ionosphere is divided into three regions whose boundaries are approximately spherical and concentric with the surface of the earth.
Please note that this is the term as it stands in the original IALA Dictionary edition (1970-1989)
Centre of buoyancy
International Association of Marine Aids to Navigation and Lighthouse Authorities - AISM