renal clearance
Imagine the kidneys as a filter that cleans a certain amount of water each minute. Renal clearance asks a precise version of this: how many millilitres of blood plasma does the kidney wipe completely free of the drug every minute? It is a rate of cleaning, not an amount of drug.
Mathematically, renal clearance equals the rate at which drug appears in the urine divided by the plasma concentration of the drug. It is the net result of three processes happening at once: filtration at the glomerulus pushes unbound drug out, tubular secretion actively adds more, and tubular reabsorption claws some back. So renal clearance = (filtration + secretion) minus reabsorption.
Comparing a drug's renal clearance to the glomerular filtration rate of about 120 mL/min is informative. A clearance well above this means secretion dominates; a clearance well below it means reabsorption is strong. Because filtration depends on kidney function, renal clearance falls in kidney disease, so renally cleared drugs accumulate and their doses must be reduced.
The renal clearance of penicillin is far higher than the glomerular filtration rate because acidic drug transporters actively secrete it; this is why probenecid, which blocks that transporter, was once given to raise penicillin levels.
Clearance above filtration rate signals active secretion.