partial agonist
A partial agonist is like a dimmer switch that can never reach full brightness, no matter how hard you turn it. It activates the receptor, but only part way, so even when every receptor is occupied the maximal effect stays below that of a full agonist.
This happens because a partial agonist has intermediate efficacy: it binds well but is less able to drive the receptor into its fully active shape. As a result it has a ceiling effect — pushing the dose higher cannot lift the response past its inherent maximum. Buprenorphine, a partial agonist at mu opioid receptors, relieves pain yet causes less respiratory depression at high doses than full agonists.
A partial agonist also behaves as a partial antagonist in the presence of a full agonist. By occupying receptors but only weakly activating them, it can pull a strong agonist's net effect down toward its own lower ceiling. This dual character makes partial agonists useful in addiction medicine and in fine-tuning receptor signalling.
Aripiprazole is a partial agonist at dopamine D2 receptors, stabilising signalling — boosting it where dopamine is low and damping it where dopamine is high.
A submaximal, self-limiting response.
Potency and maximal effect are separate ideas: a partial agonist can be more potent (act at lower concentration) yet still produce a smaller maximal response than a less potent full agonist.