heliopause

This term is a specialized scientific designation used primarily in astrophysics and planetary science. It describes a specific physical threshold rather than a tangible wall, representing the point where the outward pressure of the solar wind is balanced by the pressure of the interstellar medium.

Because it refers to a unique, singular boundary associated with a specific star, it is almost exclusively used in the singular. It is frequently paired with terms like "cross," "reach," or "traverse" when discussing deep-space probes like Voyager 1 and 2.