Replying to What class sums up your degree.

So I've noticed that almost all of my upper level university courses were just various applications of a single course that I took early on. I thought it would be interesting to see how other people boil down and describe their majors with just one class.

For example, my entire undergrad can be described as differential equations. Pretty much every class my last two years of college employed them in a big way and I spent the majority of my time solving them.