(noun) a drug that reduces excitability and calms a person. Synonyms: sedative drug, depressant, downer.
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Noun Definition 1
(adj) tending to soothe or tranquilize. Synonyms: ataractic, ataraxic, tranquilizing, tranquillizing, tranquilising, tranquillising. Examples: "Valium has a tranquilizing effect." "Took a hot drink with sedative properties before going to bed."