An isthmus (/ˈɪsθməs/ or /ˈɪsməs/; plural: isthmuses; from Ancient Greek: ἰσθμός isthmos "neck") is a narrow piece of land connecting two larger areas across an expanse of water that otherwise separates them. A tombolo is an isthmus that consists of a spit or bar.
Canals are often built across isthmuses, where they may be a particularly advantageous short cut for marine transport. The Panama Canal crosses the Isthmus of Panama, connecting the North Atlantic and Pacific Oceans; the Suez Canal connects the Mediterranean Sea and the Indian Ocean, cutting across the western side of an isthmus formed by the Sinai Peninsula. An example in the United Kingdom is the Crinan Canal, which crosses the isthmus between Loch Crinan and Loch Gilp, which connects the Kintyre peninsula with the rest of Scotland.
A strait is the sea counterpart of isthmus.