Zipf's Law
The observation that given a large sample of words used, the frequency of any word is inversely proportional to its rank in the frequency table. Therefore, the most frequent word will occur about twice as often as the second most frequent word, three times as often as the third most frequent word, etc.
Origin
French stenographer Jean-Baptiste Estoup first noted the pattern in 1916, but American linguist George Kingsley Zipf at Harvard University formalized and popularized it in his 1935 book The Psycho-Biology of Language and his 1949 Human Behavior and the Principle of Least Effort. Zipf proposed that the frequency-rank relationship reflected a universal drive toward minimal communicative effort. The law has since been found to apply far beyond linguistics, from city populations to internet traffic.