Can you find the derivative of a multivariable function?

If you have a scalar function $U(x,t)$, can you find a formula for its derivative? Or that is just what's called the differential? Or should we say it doesn't have a "derivative" in the sense of real functions of a real variable, instead it only has partial derivatives? This may be silly to some, but I wanted to have it clear in my mind. Thanks!

A function of more than one variable, say U(x, y, z, t) has partial derivatives, $\frac{\partial U}{\partial x}$, $\frac{\partial U}{\partial y}$, $\frac{\partial U}{\partial z}$, and $\frac{\partial U}{\partial t}$

It does not have a single "derivative" but does have a "differential": $dU= \frac{\partial U}{\partial x}dx+ \frac{\partial U}{\partial y}dy+ \frac{\partial U}{\partial z}dz+ \frac{\partial U}{\partial t}dt%$.