It is usually a measure of how much one variable changes relative to changes in other variable(s). The pressure of an ideal gas depends on its temperature and volume. There are rates of change for pressure
- relative to the temperature (with volume held
- relative to the volume (with temperature held constant),
- a multivariate rate of change when both can vary.
Next, the rate of change need not be constant. For a body in accelerated motion, the rate of change of location (its velocity) varies with the acceleration. And that, in turn could vary with the force acting on the body.