Isthmus of Corinth
Noun
Meaning
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
Source: WordNetExamples
Megara deserted the Spartan-led Peloponnesian League and allied herself with Athens, allowing construction of a double line of walls across the Isthmus of Corinth and protecting Athens from attack from that quarter.
The Allies prepared to defend the Isthmus of Corinth whilst the fleet was withdrawn to nearby Salamis Island.