Final answer:
An antagonist is a drug that binds to a receptor but does not activate a biological response, preventing the receptor's normal function and the initiation of signal transduction pathways.
Step-by-step explanation:
When a drug binds to a receptor but does not activate a biological response, it is typically acting as an antagonist. Antagonists prevent the normal function of a receptor without triggering the signaling pathways that would typically be initiated by an agonist binding. Receptors are protein molecules in or on target cells that bind to ligands, and the binding initiates a signaling pathway within the cell, sometimes involving a second messenger. Antagonists can inhibit these biological effects either by blocking the receptor or by indirectly preventing the signal transduction pathways from being activated, as in the case of non-competitive inhibitors. This understanding of how substances interact with receptors is key in the development of pharmaceutical drugs aimed at correcting specific neurotransmitter imbalances or dysregulated signaling pathways involved in diseases.