a relatively narrow strip of land (with water on both sides) connecting two larger land areas
source: WordNet 3.0
a cord-like tissue connecting two larger parts of an anatomical structure
source: WordNet 3.0
A neck or narrow slip of land by which two continents are connected, or by which a peninsula is united to the mainland; as, the Isthmus of Panama; the Isthmus of Suez, etc.