1. The outer boundary of the sun where the solar or interplanetary magnetic environment gives way to the interstellar: In astronomy, the heliopause is the outside edge or boundary of the heliosphere, the part of our solar system that's influenced by the sun.

The orbits of all the major planets, including Earth's, lie well within the heliopause.

Astronomers have various theories about what happens beyond the heliopause, at such a far distance from the sun that solar wind doesn't affect anything.

Inside the heliopause is the heliosphere, and everything outside of the heliopause is considered interstellar space.

2. Etymology: The word heliopause is rooted in the Greek word for "sun," helios, and the Latin pausa, "a halt, a stop, or a cessation."