[contravariant]

The first one isn't really symbolic though. So really you should compare:

- speed is the derivative of position with respect to time.

and

- Let x(t) be the position of an object at time t then its speed v(t) is:

   v(t) = (dx/dt)(t)

Also note that most of the time I'm just putting the symbols after the word explaining what it means, while this does allow me to use the symbolic notation for differentiation it doesn't really make the first part any shorter. Also it would be a mistake to introduce speed by just 1 specific formula (even if I didn't specify the types of the object involved) since speed is a far more general concept.