bridgehead

Short definition from Wiktionary

Noun

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