Antidepressant: Anything, and especially a drug, used to prevent or treat depression. The available antidepressant drugs include the SSRIs or selective serotonin reuptake inhibitors, MAOIs or monoamine oxidase inhibitors, tricyclic antidepressants, tetracyclic antidepressants, and others. They aim to correct chemical imbalances of neurotransmitters in the brain that are believed to be responsible for changes in mood and behavior.