Leonardo of Pisa, also known as Fibonacci, was an Italian mathematician born around 1170. He is best known for introducing the Hindu-Arabic numeral system to Europe, which replaced the cumbersome Roman numerals. His famous work, the Liber Abaci, demonstrated the advantages of this new system and included the now-famous Fibonacci sequence, a series of numbers where each number is the sum of the two preceding ones. Fibonacci's contributions to mathematics extended beyond just numbers. His work influenced various fields, including geometry and commerce. The Fibonacci sequence has applications in nature, art, and computer science, showcasing the interconnectedness of mathematics with the world around us.