1. The volume of space, surrounding a planet, controlled by the planet's magnetic field, and acting as a magnetic "shell": The Earth's magnetosphere extends 64,000 kilometers or 40,000 miles toward the Sun, but many times this distance on the side away from the Sun.

2. The magnetic field of a planet in space: A planet's magnetosphere is molded by the solar wind into a teardrop shape, its point directed away from the Sun.