No, it isn’t considered to be an island even though it is separated from the mainland by water.  Bridges provide permanent links between the Peloponnese peninsula and mainland. However, Lefkas and Evia are called islands although they are also linked by bridges to the mainland. The difference seems to be that the Corinth Canal that separates the Peloponnese was made by man whereas it’s the sea that separates Lefkas and Evia.