bridgehead
n//ˈbɹɪd͡ʒhɛd//brohuvud, brofäste
(military) A fortification around the end of a bridge.
(military) 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.
(physical chemistry) Either of the two atoms in different parts of a molecule that are connected by a bridge of two or more other atoms