ISTHMUS, noun ist'mus. [Latin] A neck or narrow slip of land by which two continents are connected, or by which a peninsula is united to the mainland. Such is the Neck, so called, which connects Boston with the main land at Roxbury. But the word is applied to land of considerable extent, between seas; as the isthmus of Darien, which connects North and South America, and the isthmus between the Euxine and Caspian seas.
Source: Webster's 1828 American Dictionary ↗ · public-domain · returned verbatim, no AI