Fibonacci was the nickname of Leonardo of Pisa, an Italian mathematician who was born in the 12th century. He is best known for his introduction of the Fibonacci sequence, a sequence of numbers in which each number is the sum of the two previous numbers. This sequence has many interesting mathematical properties and appears frequently in nature. Fibonacci also wrote a book called "Liber Abaci" (Book of Calculation), which introduced the Hindu-Arabic numeral system to Europe and played a major role in reviving the study of mathematics in Europe after the Dark Ages.