— exospherical /ek'seuh sfer"i keuhl, -sfear"-/ , exospheric , adj.
/ek"soh sfear'/ , n.
the highest region of the atmosphere, where the air density is so low that a fast-moving air molecule is more than 50 percent likely to escape from the atmosphere instead of hitting other molecules.
[ 1950-55; EXO- + -SPHERE ]