from The American Heritage® Dictionary of the English Language, 5th Edition.
- noun An outer region of the solar system, about 8 billion miles (12.8 billion kilometers) from the Sun, where a standing shock wave is generated as the solar wind, encountering the interstellar medium, slows to subsonic speeds.
from Wiktionary, Creative Commons Attribution/Share-Alike License.
- noun astronomy the
boundarymarking one of the outer limits of the Sun's influence, where the solar winddramatically slows
- noun astronomy by extension, that point for space around any
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