bridgehead

Meanings

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

Etymology

Calque of French tête de pont, equivalent to bridge + head.