Law of large numbers
Jacob Bernoulli first described the LLN. He says it was so simple that even the stupidest man instinctively knows it is true. Despite this, it took him over 20 years to develop a good mathematical proof. Once he had found it, he published the proof in Ars Conjectandi (The Art of Conjecturing) in 1713.  He named this his "golden theorem". It became generally known as "Bernoulli's theorem" (not to be confused with the law in physics with the same name). In 1835, S.D. Poisson further described it under the name "la loi des grands nombres" (the law of large numbers).  Thereafter, it was known under both names, but the "law of large numbers" is most frequently used.