A second language is any language consciously learned or used by its speaker after puberty, according to linguist Eric Lenneberg's "Critical Period Hypothesis". In most cases, people never achieve the same level of fluency and comprehension in their second languages as in their first, or native, 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), hence why the Latin version of the Bible is called the Vulgate.
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 America in the fields of business and entertainment. French was for a time the lingua franca 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 colonisation. French continues to be one of the world's most widely spoken languages.