Leonhard Euler (pronounced OY-ler, not YOO-ler) was a Swiss mathematician (among other things) so good at his study that there is an entire Wikipedia page dedicated to the things named after him! (Link). It’s hard to navigate any field of modern science without accidentally running into his work.
Perhaps the most widely popular part of Euler’s work would be Euler’s Constant, e (approx. 2.71828). (To be fair, it wasn’t named after him until long after he died.) Euler’s constant is an irrational number known for having fantastic properties and appearing in an extremely broad amount of topics.
The constant’s most popular application is in calculus, where the function is its own derivative. This is also the only function in math to have this astonishing property!
Among mathematicians, the aptly named Euler’s Identity is perhaps the most novel place that e appears, for it is widely regarded to be the most beautiful equation in all of mathematics.