Italian Peninsula - a boot-shaped peninsula in southern Europe extending into the Mediterranean Sea Italia, Italian Republic, Italy - a republic in southern Europe on the Italian Peninsula; was the core of the Roman Republic and the Roman Empire between the 4th century BC and the 5th century AD Republic of San Marino, San Marino - the smallest republic in the world; the oldest independent country in Europe (achieved independence in 301); located in the Apennines and completely surrounded by Italy