Wiktionary
df
n. (alternative form of DOF English) (In the sense ''degree of freedom'')
Wikipedia
DF
DF or df may refer to:
Df (Unix)
(abbreviation for disk free) is a standard Unix command used to display the amount of available disk space for file systems on which the invoking user has appropriate read access. is implemented using the statfs or statvfs system calls.
first appeared in Version 1 AT&T UNIX.