Isthmus - n. - A narrow strip of land with sea on either side, forming a link between two larger areas of land.

As seen in [Panama](https://upload.wikimedia.org/wikipedia/commons/4/4b/IsthmusOfPanama.png) and [Madison, WI](https://ecsjets.com/web_images/content/madison.jpg). Both isthmuses and isthmi are acceptable plural forms.