Selective serotonin reuptake inhibitor: A selective serotonin reuptake inhibitor (SSRI) is one of the commonly prescribed drugs for treating depression. SSRIs affect the chemicals that nerves in the brain use to send messages to one another.elective serotonin reuptake inhibitor: A selective serotonin reuptake inhibitor (SSRI) is one of the commonly prescribed drugs for treating depression. SSRIs affect the chemicals that nerves in the brain use to send messages to one another.