This is more nitpicking. Yes, there's a difference between partition and disk. But if we want to get technical, it's not disk encryption unless you're using a HDD. SSDs don't have disks.

At the end of the day, FDE would generally imply that all partitions with user data on them are encrypted. So it would generally include root and home partitions, and generally not include the boot partition, and would likely include partitions like /var and /opt, though not necessarily.