This is probably of somewhat specialized interest, but it seems worth passing along. Roughly speaking, stochastic calculus is the extension of the methods of ordinary calculus from normal variables to random processes. It’s foundational to modern financial theory, widely used in cellular biology and particle physics, and probably has applications that I’m not aware of.
So that was the mother fucker who damned near terminated my grad school career. Fucking A, I got a B- in an undergraduate statistical signal processing class and was about 0.07 points above academic probation. To this day, I’m still not sure what a stochastic process is.