A Unix-like operating system is one that behaves in a manner similar to the UNIX system. The first Unix-like operating systems were developed because of AT&T's licencing of Unix, which prevented the sale of Unix to commercial organisations. The Unix-like operating systems that were available in the early 1980s included Idris, Coherent, UniFlex, and Minix. When AT&T allowed Unix to be sold to commercial organisations the market for the Unix-like operating systems dried up.
The Open Group, which owns the UNIX trademark, considers "UNIX-like" to be a usage problem, and believes that a better alternative term would be the term "POSIX-conforming system". However, the term POSIX-conforming system has its own problems. For example, there are systems which conform to at least the key POSIX standards (e.g., Microsoft Windows) and can claim to be POSIX-conforming, but nevertheless they are not Unix-like and thus many applications written to run on Unix will not run on them.
This term is often used to refer to the open source Unix-like systems: