Tranquillizer/ˈtræŋkwəlaɪzər/ Definition 1 of 2Pro Tip 1/3Medical Use Used in healthcare to describe medications for anxiety or insomnia. The doctor prescribed a tranquillizer for her severe anxiety. Pro Tip 2/3Veterinary Setting Often used by vets to sedate animals for treatment or transport. The lion was given a tranquillizer before being moved to another zoo. Pro Tip 3/3Calm Emotions May also refer to reducing psychological stress or excitement. After the traumatic event, he needed a tranquillizer to sleep.