Noun, pl. isthmi, isthmuses
1. (Anatomy) Any such narrow part connecting two larger structures. (wiktionary.org)
2. A narrow organ, passage, or piece of tissue connecting two larger parts. (Google Dictionary)
3. (Geography) 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. Isthmus of the fauces. (biology-online.org)
Word origin: From Greek isthmos “narrow passage, narrow neck of land,” especially that of Corinth, of unknown origin, perhaps from eimi “to go” + suffix –thmo (cf. ithma “a step, movement”).