Simeon Denis Poisson, born on June 21, 1781, was a French mathematician known for his contributions to various fields of mathematics, including probability theory, electromagnetism, and celestial mechanics. He made significant advancements in the study of definite integrals and the theory of probability. Poisson's work has had a lasting impact on the development of mathematical physics and has been widely applied in various scientific disciplines. His contributions to the field of mathematics have earned him recognition as one of the most influential mathematicians of the 19th century.