Many standard calculus texts use r as the default function name when defining vector-valued functions, e.g.,
$\textbf{r}(t)=\langle x(t),y(t),z(t)\rangle$.
For scalar-valued functions, we default to f, which makes sense because it is the initial letter of the word "function". However, I can not think of an analogous reason as to why we might use r for vector-valued functions.
Does anyone have an insight on the history of this convention?