To make a person or an animal calm — or unconscious — by giving them a drug. In real use it is overwhelmingly about animals: vets and wildlife teams tranquillize a bear, a tiger, an escaped horse, usually with a dart fired from a gun so they can move or treat the animal safely. Used of people it sounds clinical and slightly disturbing, because it suggests the person was drugged into calmness rather than choosing it, and it often carries criticism — patients tranquillized to keep the ward quiet. The related noun 'tranquillizer' is the everyday word here: it means a drug that calms someone, and 'tranquillizer dart' is what most people actually say. There is a rarer, gentler use with no drug involved at all, where writers say a sound or a place tranquillizes the mind, meaning it soothes it — that is literary and you will not need it in speech. British English spells it with two l's and often -ise (tranquillise); American English writes 'tranquilize'.

（用藥物）使鎮靜、麻醉（動物常說「打麻醉槍」；藥物本身叫「鎮靜劑」）