What is an isthmu?

Answer 1

Isthmus of Suez, Isthmus of Panama :D

Answer 2

An Isthmus is a narrow strip of land connecting two larger land areas usually with water on either side. Of note, the Isthmus of Corinth connects the peninsula of Peloponnese with the rest of the Greek peninsula, the Isthmus of Panama connects the continents of North and South America (the Americas), and the Isthmus of Suez in Egypt connects Africa and Asia (or Eurasia).

Canals are often built on isthmuses where they may be particularly advantageous to create a shortcut for marine transportation. The Panama Canal bisects the Isthmus of Panama, thereby connecting the North Atlantic and Pacific Ocean; the Suez Canal adjoins the North Atlantic and Indian ocean, cutting across the Isthmus of Suez to connect the Mediterranean Sea and the Gulf of Suez/Red Sea. A strait is the sea counterpart of isthmus.
An isthmus is a "land bridge" bordered on both sides by water that connects two larger bodies of land.
A narrow strip of land with sea on either side. (dictionary.com)