An isthmus is a land bridge, a narrow strip of land that stretches across a body of water to connect two larger land masses.

The Greek isthmos means "neck," so you can see how isthmus came to mean the connecting strip between land masses. If you look at an example, say the Isthmus of Panama, you can see how that neck-like bit of land separates the Caribbean Sea from the North Pacific Ocean as it connects North and South America. Think of an isthmus as kind of like a turkey neck that connects the bird's body and head (at least until Thanksgiving).

Definitions of isthmus

n a relatively narrow strip of land (with water on both sides) connecting two larger land areas

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Central America
the isthmus joining North America and South America; extends from the southern border of Mexico to the northern border of Colombia
Isthmus of Corinth
a narrow isthmus between the Gulf of Corinth and the Saronic Gulf; a canal crosses the isthmus so that navigation is possible between the gulfs
Isthmus of Kra
a narrow isthmus linking the Malay Peninsula to the Asian mainland
Isthmus of Darien
the isthmus that connects Central America and South America; was formerly called the Isthmus of Darien
Isthmus of Suez
the isthmus in northeastern Egypt that connects Africa and Asia
Isthmus of Tehuantepec
the narrowest part of southern Mexico is an isthmus between the Bay of Campeche on the north coast and the Gulf of Tehuantepec on the south coast
Karelian Isthmus
the isthmus between the Gulf of Finland and Lake Ladoga that connects Finland and Russia
Type of:
dry land, earth, ground, land, solid ground, terra firma
the solid part of the earth's surface

n a cord-like tissue connecting two larger parts of an anatomical structure

Type of:
part of an organism consisting of an aggregate of cells having a similar structure and function