Latin (language)
Latin was the language spoken by the ancient Romans. (Rome was in the district called Latium in Latin, and 'Latin' originally meant simply 'from Latium'.) The Romans famously had the greatest empire in Europe over at least the first four centuries of the Common Era.
The Latin language is of great importance in several ways. Historically, the language of the Roman Empire was obviously important, and it was then associated with the Christian religion which developed within it, eventually becoming the only official religion of the Empire.
Throughout Europe, it was the language of Higher Education from the time of the Roman Empire until about the 18th century; and the language of scholarship even later than that. It was also the language of the Christian religion in Western Europe until the Reformation in the 16th century. The old form of Western European Christianity (the Roman Catholic church) only stopped using Latin as the only language in church services in the 1960s, under the Second Vatican Council. (Latin is still the official language in the Vatican, the Pope's state within the city of Rome.) These two circumstances of its use (education and religion) made the language hugely important for 2000 years.