A second language is any language consciously learned or used by its speaker after infancy. In most cases people never achieve the same level of fluency and comprehension in their second languages as in their first, or native, language.
The greatest second language in Europe was Latin, as it was used by the Church and by the Law (as it still is today). It was also used in Medicine (but that was 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 (i.e. common language), hence why the Latin version of the Bible is called the Vulgate. This is not bad to what could have been considered a Dead Language.
Nowadays English is the most widespread second language, used on subjects as diverse as the Internet, TV, Radio etc. This is due in no small part to America's big part in these fields. French comes a close second, due to French's big part in the world, also its big part in the food industry.