A psychopharmacologic drug that tends to calm overexcited individuals, producing a state of “tranquility.” The term tranquilizer was originally applied to two groups of drugs. Members of one group, the antianxiety drugs, were called minor tranquilizers, and members of the second group, the antipsychotic drugs, were called major tranquilizers. Although drugs in both groups may have a calming effect in appropriate doses, it is now clear that the two groups are quite different.