Definition of THERMOSPHERE. : the part of the earth's atmosphere that begins at about 50 miles (80 kilometers) above the earth's surface, extends to outer space, and is characterized by steadily increasing temperature with height. — ther·mo·spher·ic \ˌthər-mə-ˈsfir-ik, -ˈsfer-\ adjective.ADVERTISEMENT. the part of the earth's atmosphere that begins at about 50 miles (80 kilometers) above the earth's surface, extends to outer space, and is characterized by steadily increasing temperature with height. — ther·mo·spher·ic \ˌthər-mə-ˈsfir-ik, -ˈsfer-\ adjective. ADVERTISEMENT.