A lingua franca, according to most definitions, is a language used to communicate between people who speak different languages natively. Because there are different ideas people want to communicate, different languages fill this role. Currently, English is the language which fills the most roles.
The definition of lingua franca accepted by most sources is a common language used by people who normally speak other languages. While the words actually mean 'Frankish language', from the Italian language, the concept is much older than Italian, or even Latin.
Before the rise of Rome, Greek was the language of educated men, from the philosophers of Greece to Alexander the Great. Because Alexander conquered most of the 'known' world, most important documents were copied in Greek, even if they were written in other countries.
As the Roman Empire began to conquer the world, Latin spread where ever the legions went. For a while, educated Roman citizens had their children taught by Greek tutors, and even after the fall of Rome to barbarians in the fifth century, the Byzantine Empire, which saw itself as the surviving Rome, kept Greek alive as an essential language.
Latin, however, had spread far beyond where Greek had gone, and as the language of the Catholic church, stayed of primary importance. Monasteries had precious libraries of Latin scrolls and then books, and it was the language of scholarship. When universities were established in the middle ages, the men of many countries who attended them were taught in Latin, and they wrote the fruits of their scholarship in Latin as well. Even today, Latin is not only the lingua franca of the Catholic church, but is the official language of the Vatican.
Arabic was a lingua franca among the counties in the Islamic Empire, till the fall of the Ottoman Empire, and was used by all those who traded with the Islamic Empire. It is still the lingua franca of the Mid East. Arabic is also a language gaining in use on the Internet.
Later Lingua Franca
German, or a form of it, was the lingua franca of the Holy Roman Empire, from the time of Charlemagne to the 16th century. After that, it was still considered the language of science well into the beginning of the 20th century, along with some remnants of Latin nomenclature. Some scientific literature is still published simultaneously in English and German. After WWII, German became less politically correct, and more and more work which would have been published only in German was published jointly in German and English, or only in English.
French was the preeminent language of diplomacy from the 17th century until the Treaty of Versailles, in the 20th century, when the Treaty was written both in English and French. It was used internationally for so many purposes that it is still of great importance in international organizations, and of the six languages that are the official languages of the United Nations, it is French and English in which most international documents are written. French was also the language of literature and letters as Latin grew less used.
Spanish grew in importance in the period of world colonization, and still is a lingua franca throughout most of Central and South America, as well as some of the other former Spanish colonies in Africa and some of Asia. As well, it continues to become of greater importance in the United States, as a growing percentage of the American population speaks Spanish as a native tongue.
Russian was the lingua franca of the USSR, of the Soviet Union - used throughout all the different countries in the USSR. Since the breakdown of the Soviet Union, the number of official speakers has dropped drastically, although that may also be a political decision. It is no longer accepted as the sole lingua franca of the former Soviet Union, and many countries have resorted to English rather than use Russian to communicate between different nations. Some scientific material is also published simultaneously in both Russian and English, and Russian continues to be a presence on the Internet.
Chinese, or its various dialects, was the lingua franca of Asia, and again is as important as it was in the time of the Mandarin Empire. It also is spoken natively by more people than any other language. Through the sheer numbers of people speaking Mandarin and other Chinese dialects, it is growing in significance as a language on the Internet.
Hebrew, which is not a language spoken by a huge portion of the world's inhabitants, remains a lingua franca, as it has since the time of the Romans, and the diaspora of the Jews. It spread throughout Europe and the Middle East as a way for Jews in different countries to communicate with one another despite the languages of the many countries where they were born. It has also stayed important in its role as a lingua franca because of its religious significance.
The New Lingua Franca
As the British Empire grew, the English language grew in importance, spread all over the world by the many British colonies, it stayed as an official language in many of them even after the colonies were their own countries again. At the Treaty of Versailles, it became officially one of the 2 languages of diplomacy when the Treaty was written in both French and English.
Early in the 20th century, it became the language where most scientific research was published- although many papers continued to be published in the native language of the scientist as well. Now, to get cataloged internationally, a paper needs to be available in English, whatever the native language of the scientist.
While it is only the third most commonly spoken native language, English has become by far the most commonly understood language in the world today. It is estimated by the SLI Ethnologue that some 1800 million people speak some English, although nowhere near that number approach fluency.
It is the language of business, science, politics, news, information, entertainment and the arts, and even gaming and although other languages are gaining usage on the Internet, it is still the common tongue. It is truly the global lingua franca of the 1st century.
In many ways, it is so prevalent that other less spoken languages are going extinct with greater and greater frequency.