Operating systems like Unix, including Unix. This generally includes every operating system except DOS/Windows, BeOS, MacOS, and VMS. Examples include System V, Linux, and BSD. They all have CLIs and are generally secure and stable.

I think the NetBSD project put the reasoning for this term quite well:

If something looks like a duck, walks like a duck, and quacks like a duck, what is it?

The answer, of course, depends on whether or not the name `duck' is a trademark! If it is, then the closest that something can get, without permission of the owner of the trademark, is `duck-like.'

`UNIX' is a trademark of The Open Group, and NetBSD has not been branded with that trademark. Therefore, NetBSD is not UNIX. We refer to it as `UNIX-like' or `UN*X-like.'

(not to mention if the duck comes from some of the same code..)

Update: yerricde points out http://www.unix-systems.org/questions_answers/faq.html, which says:

What is a "Unix-like" operating system?

This term, which is an abuse of the UNIX trademark, and should not be used, is often used in association with products that share some properties in common with systems carrying the UNIX trademark. Users of such terms should consult the trademark usage guidelines, which give numerous examples of how to correctly use the trademark, and make the necessary corrections.

I for one strongly disagree with this claim of trademark abuse. It's like saying people can't express something as like another product. But IANAL.