A narrow piece of land connecting two large masses of land :

One word substitute is Isthmus


Peninsula: a piece of land almost surrounded by water or projecting out into a body of water.
Isthmus: a narrow strip of land with sea on either side, forming a link between two larger areas of land.
Continent: any of the world's main continuous expanses of land (Europe, Asia, Africa, North and South America, Australia, Antarctica).
Gulf: a deep inlet of the sea almost surrounded by land, with a narrow mouth.