Fibonacci, also known as Leonardo Bonacci, Leonardo Fibonacci and Leonardo of Pisa, lived c. 1170–1250. He was an Italian mathematician. He was thought "the most talented Western mathematician of the Middle Ages".
Fibonacci popularized the Hindu–Arabic numeral system to the Western World. He did this in his composition in 1202 of Liber Abaci (Book of Calculation). He also introduced to Europe the sequence of Fibonacci numbers which he used as an example in Liber Abaci.
Fibonacci number sequence
Fibbonacci is best known for the list of numbers called the Fibonacci Sequence. The list never stops, but it starts this way:
1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, ...
In this list, a person can find the next number by adding the last two numbers together.
1 + 1 = 2 1 + 2 = 3 2 + 3 = 5 3 + 5 = 8 5 + 8 = 13 8 + 13 = 21 13 + 21 = 34 21 + 34 = 55 34 + 55 = 89 55 + 89 = 144 89 + 144 = 233 144 + 233 = 377 233 + 377 = 610 377 + 610 = 987 610 + 987 = 1597 987 + 1597 = 2584 etc...
This series is also interesting because the ratio of two adjacent numbers in the series approaches the golden ratio.
Images for kids

A page of Fibonacci's Liber Abaci from the Biblioteca Nazionale di Firenze showing (in box on right) the Fibonacci sequence with the position in the sequence labeled in Roman numerals and the value in HinduArabic numerals.