a narrow neck, or slip of ground, which joins two continents; or joins a peninsula to the terra firma, and separates two seas. See PENINSULA.

The most celebrated isthmuses are that of Panama or Darien, which joins North and South America; that of Suez, which connects Asia and Africa; that of Corinth, or Peloponnesus, in the Morea; that of Crimean-Tartary, otherwise called Taurica Chersonesus; that of the peninsula Romania, and Erifio, or the isthmus of the Thracian Chersonesus, twelve furlongs broad, being that which Xerxes undertook to cut through. The ancients had several designs of cutting the isthmus of Corinth, which is a rocky hillock, about ten miles over; but they were all in vain, the invention of sluices being not then known. There have been attempts too for cutting the isthmus of Suez, to make a communication between the Red sea and the Mediterranean; but these also failed; and in one of them a king of Egypt is said to have lost 120,000 men.