A Unix-like (sometimes shortened to *nix to circumvent trademark issues) operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification.
There is no standard for defining the term, and some difference of opinion is possible as to the degree to which a certain OS is "Unix-like".
The term can include free and open source operating systems inspired by Bell Labs' Unix or designed to emulate its features, commercial and proprietary work-alikes, and even versions based on the licensed UNIX source code (which may be sufficiently "Unix-like" that they pass certification to bear the "UNIX" trademark).
Free and open source examples are sometimes known as Freenix.[1]