the highest region of the ionosphere (from 90 to 600 miles up) which contains the highest concentration of free electrons and is most useful for long-range radio transmission | a part of an animal that has a special function or is supplied by a given artery or nerve |
| a knowledge domain that you are interested in or are communicating about |
| the extended spatial location of something |
| a large indefinite location on the surface of the Earth |
| the approximate amount of something (usually used prepositionally as in `in the region of') |