Briefly: A breach in a corner is a breach located at a corner. Such a breach invalidates the eruv at a measure of 32 cm.
A breach up to ten amos (4.8 meters) can be considered a doorway if the standing section equals the breached section. However, since people do not normally make doorways in corners, whenever there is a breach “in a corner” measuring four tefachim diagonally (and some say three tefachim), the area is considered breached and must be repaired.
“Keren zavis” means that the breach “eats into” both adjoining sides of the eruv, not into a single direction. If one were to extend the remaining wall, it would not reach the opposite wall; rather, the area would remain open to the outside.
There is a disagreement regarding a circular, hexagonal, octagonal, or any non-straight fence or boundary, whether a breach in such a structure is considered a breach in a corner.
A breach in a corner can be repaired with a tzuras hapesach.
