Apparently most simplist differential equation is retarded.

I was doing work things and I end up seeing a math paper that said something retarded function something. Seconds later, wait WHAT function! So yeah, according to Introduction to functional differential equations by Jack K. Hale, and Sjoerd M. Verduyn Lunel retarded differential function is: $$\frac{\partial x\left(t\right)}{\partial t}=f(t,x\left(t+\theta \right))$$ Functions $x$ and $f$ output vectors and $x$ seems to have to be from Banach space of continuous functions and there is some other (possible?) limitation around $t$ and $\theta$. The fact that this is called retarded makes me so happy. The name may come from the fact that the argument of the function is retarded? A couple of papers referred to retarded argument but I didn't find the definition. Introduction to functional differential equations doesn't reference it, so no idea... The book is interesting though. Have to add to the reading pile.