clearance
Clearance measures how efficiently the body gets rid of a drug. Rather than counting molecules, it asks: what volume of blood plasma is completely scrubbed clean of the drug each minute? If a drug's clearance is 100 milliliters per minute, the body is effectively wiping the drug out of 100 milliliters of plasma every minute, even though in reality it only partly removes drug from a much larger volume.
Clearance is a property of the eliminating organs — mostly the liver, which metabolizes drugs, and the kidneys, which excrete them. Total clearance is the sum of these organ clearances. Crucially, clearance is an intrinsic measure of elimination capacity: it does not depend on how much drug is present, only on how good the organs are at removing it.
Clearance is one of the two pillars of pharmacokinetics, alongside volume of distribution, and together they set a drug's half-life. It also determines the maintenance dose needed at steady state: to keep blood levels constant, the rate you put drug in must equal clearance times the target concentration. A drug with high clearance must be dosed more often or in larger amounts to maintain its effect.
If a drug's clearance is 6 L/h and you want to hold its plasma concentration at 2 mg/L, you must infuse it at 12 mg per hour, because input rate at steady state equals clearance times target concentration.
Clearance directly fixes the maintenance dose rate.
Clearance has units of volume per time (e.g. mL/min or L/h) — it is a flow, not an amount. Together with volume of distribution it determines half-life, but it is the more fundamental of the two.