potency
Potency answers a practical question: how much drug do you need? A more potent drug achieves a given effect at a smaller dose, like a stronger coffee that wakes you with a single sip rather than a whole pot.
Potency depends mainly on affinity (how tightly the drug binds) and on how efficiently the receptor system translates binding into response. It is usually summarised by the EC50 or ED50 — the concentration or dose giving half the maximal effect. A leftward-shifted dose–response curve means lower EC50 and therefore higher potency.
Potency is often confused with efficacy, but they answer different questions. Potency is about the dose needed; efficacy is about how big an effect is ultimately possible. A very potent drug that produces only a weak maximal response may be less clinically useful than a less potent one that can reach a full effect. In practice, modest differences in potency rarely matter, since the dose can simply be adjusted.
Fentanyl is roughly 100 times more potent than morphine: micrograms of fentanyl match milligrams of morphine, though both are full opioid agonists.
Less drug for the same effect.
A lower number means higher potency: a drug effective at 1 mg is more potent than one needing 100 mg, even if both reach the same maximal effect.