The Corinth Canal cuts through the narrow Isthmus of Corinth and separates the Peloponnesian peninsula from the Greek mainland and therefore effectively making the former an island.
While technically it may be considered an island since the construction of the Corinth Canal in 1893, like other peninsulas that have been separated from their mainland by man-made bodies of waters, it is rarely, if ever referred to as an "island".
It has two land connections with the rest of Greece, a natural one at the Isthmus of Corinth and an artificial one in the shape of the Rio-Antirio Bridge (completed 2004).
Here's a ticket to one of the objects :) Just to let you know what it looks like.
Don't pay much attention to the price, my wife pays full price and I'm a student so I pay only half of it - and I don't remember whose ticket this one was ;)))