What does Peninsula from ancient Greece mean?

A Peninsula is a piece of land that is surrounded by water but is connected to land, If you look at a map of Greece, the Peninsula begins near the city of Corinth, Sparta is located on the Peninsula and Athens is located on the main land.

Here is a link to a map that shows the peninsula, the small passage or land near east of Corinth is the important part which defines it as a peninsula instead of an Island.