The phrase "narrow isthmus" refers to a thin strip of land that connects two larger bodies of land. It suggests that the isthmus is not very wide, making it a narrow passage between two bodies of water or larger landmasses. Full definition
Central America is a very narrow isthmus of southern North America extending from Mexico to Panama. (travel.padi.com)
Located on the Caribbean coast, Placencia is called «The Island You Can Drive To» because of it's laid - back charm, gorgeous beaches, and narrow isthmus connecting it to the mainland. (chabilmarvillas.com)
The standout feature, La Coupee, is a long and narrow isthmus reaching 250ft high, linking the two main islands which once represented a precarious hiking route until safety fences were erected in 1900. (directferries.com)