The rate of a process is the speed at which it occurs. For example the rate of drug absorption is the amount of the drug absorbed per unit time, and the rate of drug elimination is the amount of the drug eliminated per unit time.

The rate constant is the constant that determines the rate of the process. The way the rate of the process is determined from the amount of the drug involved in the process and the rate constant of the process is dependent on the order of the process.

The order of the process determines how the amount of the drug involved in the process will influence the rate of the process.