Actually, this is an incorrect term. While Linux is in fact a clone, BSD and SunOS are not, because they contain code derived from the original AT&T UNIX.
GNU, Linux, and
Minix are UNIX clones. Everything else I can think of are not. Though
legally, many are not
UNIX, because of
The UNIX Trademark. Thus, the best term is probably "
UNIX-like operating system", to
avoid being
sued. But they are by no means clones.