Clearance (CL)
CL describes the relationship between concentration and the rate of elimination of drug from the body. Note that elimination and clearance are NOT the same thing.
On a related note, the fact that CL is made-up and has no physical meaning is an important concept to understand. CL is more a consequence of the underlying mathematics of pharmacokinetics than a real biological process. What’s more, CL is near impossible to interpret on its own, but somehow is still common to see in the summary of product characteristics for approved drugs.
A more intuitive parameter is the half-life (t1/2), which is the time it takes for the concentration of a drug to decrease by half. However, even if the half-life is more intuitive, it is not very useful during the current state of modern drug developement. The half-life of a drug is really only useful if a drug follws one-compartment pharmacokinetics. It is rare for modern drugs to follow one-compartment pharmacokinetics.