Leonhard Euler was an 18th-century Swiss mathematician and physicist, known for his significant contributions to various fields, including calculus, graph theory, and number theory. He introduced many modern notations, such as the function notation f(x) and the concept of a mathematical constant, denoted as e, which is essential in calculus. Euler's work laid the groundwork for many areas of mathematics and science. He also made advancements in mechanics, optics, and astronomy. His prolific output includes over 800 publications, making him one of the most productive mathematicians in history.