bridgehead

noun4

1. •
   An area around the end of a bridge.
2. •
   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.
3. •
   A fortification around the end of a bridge.
4. •
   Either of the two atoms in different parts of a molecule that are connected by a bridge of two or more other atoms