The Isthmus of Corinth is the narrow land bridge which connects the Peloponnese peninsula with the rest of the mainland of Greece, near the city of Corinth. The Isthmus was known in the ancient world as the landmark separating the Peloponnese from mainland Greece. In the first century AD the geographer Strabo noted a stele on the Isthmus of Corinth, which bore two inscriptions. One towards the East, i.e. towards Megara, reading: "Here is not Peloponnesus, but Ionia" and the one towards the West, i.e. towards the Peloponnese: "Here is Peloponnesus, not Ionia" ; Plutarch ascribed the erection of the stele to the Attic hero Theseus, on his way to Athens.

The Peloponnese, Peloponnesus or Morea is a peninsula and geographic region in Southern Greece, and the southernmost region of the Balkans. It is connected to the central part of the country by the Isthmus of Corinth land bridge which separates the Gulf of Corinth from the Saronic Gulf. From the late Middle Ages until the 19th century, the peninsula was known as the Morea, a name still in colloquial use in its demotic form.