The outer region of the Earth's atmosphere; contains a high concentration of free electrons WordNet 3.0 & Open English WordNet

The ionosphere is the ionized part of the upper atmosphere of Earth, from about 48 km to 965 km above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. Wikipedia

A region of the earth's atmosphere, extending from about 60 to 1000 kilometers above the earth's surface, in which there is a high concentration of free electrons and ions formed as a result of ionizing radiation entering the atmosphere from space. OmegaWiki

The part of the Earth's atmosphere beginning at an altitude of about 50 kilometers (31 miles) and extending outward 500 kilometers (310 miles) or more. Wiktionary

Part of Earth's atmosphere. Wiktionary (translation)