The assumption, most prevalent on the historical Internet, that you're getting on the Internet using UNIX. Not very prevalent anymore; nowadays the MicrosoftWindowsCulturalAssumption is much more popular. This page is mostly speaking of unix from the console command-line.
Individuals immersed in UnixCulturalAssumption may be condescending to others in the computing field who lack fundamental understanding of unix. They may also be irritated when working in environments that do not support the power and flexibility of command-line unix.
Seems to me that this term could also be applied when you download a piece of OpenSource software and the instructions assume you are running on a UnixLike system. Although I have both Windows (95, 98, NT) and Linux platforms, it still pisses me when I spend the time to download something for my Windows laptop and the instructions read
"configure; make; make install" to install.
But isn't this really a case of your MicrosoftWindowsCulturalAssumption being at fault?
Those instructions are incomplete. They omitted the Requirements and Troubleshooting section.
Requirements
Unix.
Troubleshooting
Q: I am using Microsoft Windows and cannot do "configure; make; make install" to install.
A: This software package requires Unix. Please install Unix and try again.
I have found it helpful to write "THIS WILL NOT WORK UNDER MICROSOFT WINDOWS" in the largest font available on the web page that offers the downloads. But I still routinely get e-mail from Windows users who don't understand why InstallShield doesn't pop up when they double click on the source tarball.
It wouldn't be so bad if I could actually use CygWin (which is a pretty UnixLike subsystem for Windows) to install the software, but usually the software itself contains too many UnixCulturalAssumptions.
More and more open-source programmers are starting to support CygWin as an acceptable kind of UNIX. Most of the packages I've tried build and install fine on CygWin, as long as the author has used a recent version of autoconf. Some examples of this include SquidCyg? but it's hard to find a working source that works on a given build of cygwin
CygWin - don't leave home without it. I started installing CygWin, what, 4?, years ago. If I sit down at a development desk that doesn't have it installed, it will magically install itself. Sort of a CygWin CropCircles? phenomenon.
CygWin has quite likely also got much better. So have api's in general, one would hope.
See also about "Unix culture":
See also UnixAndWindowsHell, GrossDeficienciesOfUnix.