It is in the unit of
ZiB (or simply
1 ZiB = 2^70 Bytes
So as you have 5.0Z, multiply it by
5 to get your FS size.
2^70 = 1,180,591,620,717,411,303,424.
ZB is generally called ZettaBytes (10^21 = 1,000,000,000,000,000,000,000), but
ZiB is not given any particular name.
But if we follow the trend, e.g.
KB for KiloBytes, whereas
KiB is called KibiBytes, similarly this can be named something like ZettiBytes? Again nothing official AFAIK.
Regarding the weird filesystem size:
Check the entry carefully:
/dev/sdc 5.0Z 5.0Z 0 100% /var/sdc
you have created filesystem on the whole disk
/dev/sdc, not on any partition of it (e.g.
/dev/sdc1, have you created any?), leading to this weirdness. Unmount the filesystem, create necessary partition(s) on the disk, and mount the partition(s) (e.g. could be
/dev/sdc1) accordingly to meet your need.