a person or thing that tranquilizes; esp., a depressant drug, as Valium or Librium, used as a calming agent in relieving and controlling various emotional disturbances, anxiety neuroses, certain psychoses, etc.
See tranquilizer in American Heritage Dictionary 4
also tran·quil·liz·er(trăngˈkwə-līzˌər, trănˈ-)
noun
One that serves to tranquilize, as soothing music.
Any of various drugs used to reduce tension or anxiety; an antianxiety agent.
Any of various drugs used to treat psychotic states; an antipsychotic drug. Not in scientific use.