A second language is any language other than the first, or native, language learned; it is typically used because of geographical or social reasons. The term is to be distinguished from foreign language; linguist Eric Lenneberg uses second language in his critical period hypothesis to mean a language consciously learned or used by its speaker after puberty. In most cases, people never achieve the same level of fluency and comprehension in their second languages as in their first language.
Historically in Europe, the most widely used second language (or lingua franca) was Latin. It was used by the Church; by the Law (as it still is today); in Medicine (starting much later); Horticulture and biological classification of plants, animals, fruits, nuts, etc.
Latin was used so much across Europe that it was called the vulgar (or common tongue); this is why the Latin version of the Bible is called the Vulgate.
Nowadays, English is considered the world's most widespread second language; it is used in areas as diverse as the internet, television and radio, and international aviation.
The success of English throughout the world stems from two major causes: the far reaching influence of the British Empire, and the 20th century (and continuing) dominance of the United States in the fields of business and entertainment.
French was for a time the lingua franca (the origin of term) in Europe. In history, both England and France were ruled by a single crown — the language used by the royal court was French (English was considered "the peasant's language"). Afterwards, as was the case with English, the French empire spread its language through colonization. French continues to be one of the world's most widely spoken languages.
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