a narrow neck, or lip 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 Cram-Tartary, otherwise called Taurica Cheronea; that of the peninsula Romania, and Erifio; or the isthmus of the Thracian Cheronea, 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.