A written order from your primary care doctor for you to see a specialist or get certain medical services. In many Health Maintenance Organizations (HMOs), you need to get a referral before you can get medical care from anyone except your primary care doctor. If you don’t get a referral first, the plan may not pay for the services.

References

https://www.medicare.gov/glossary/r.html
https://www.healthcare.gov/glossary/referral/