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:

(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:

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.

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