Difference between "private" and "internal" project visibility

What's the difference between "private" and "internal" project visibility?

The /help/public_access/public_access page explains the difference between "public" and "internal", but doesn't mention "private" beyond saying • Change "Visibility Level" to either Public, Internal or Private.

Could someone please explain the difference, or point me to documentation I might have missed? Could that explanation also be added to /help/public_access/public_access?

Thanks.