Over the years, the Unix operating system
has grown into a family of probably over a hundred different variants, if we count different versions. Anyone tends to refer to any of these versions as "Unix", but this term is actually a legal brand name
, reserved to those variants that actually pass the strict technical Unix conformance tests imposed by the brand name holder.
It is not just a matter of technically conforming to the specs, you have to officially certify as an official Unix, which takes more time and money than the typical Unix variant developer finds worthwhile, not to mention that the value being conformant to some "official" Unix standard is questionable.
So in order to be legally correct, most things usually called Unix have to called by some other name. Unix-like OS is used, but a little long. "Unix variant" doesn't seem to do any harm.