bridgehead

noun

1. An area around the end of a bridge.
2. A fortification around the end of a bridge.
3. An area of ground on the enemy's side of a river or other obstacle, especially one that needs to be taken and defended in order to secure an advance.

   “He believed the invasion would happen elsewhere and it wasn't until late in the afternoon and long after the Allied forces had established a bridgehead that Hitler was convinced of the need to do something.”

4. (physical)Either of the two atoms in different parts of a molecule that are connected by a bridge of two or more other atoms