You could also say that 2x is the derivative of x squared plus pi. I think you get the general idea. If you wanted to write it in the most general sense, you would write that 2x is the derivative of x squared plus some constant. This is what you would consider the antiderivative of 2x. That's all nice, but this is kind of clumsy to have to write a sentence like this, so let's come up with some notation for the antiderivative. The convention here is to use a strange-looking notation. It's to use a big, elongated, S-looking thing like that, and a dx around the function that you're trying to take the antiderivative of.