In the past couple of weeks, I've heard several of my colleages refer to Open Source as Open Systems. I thought that this was a slip of the tongue, but since it's happened several times and by people of sufficient ... experience ... to remember Open Systems (as I do), I'm not so sure that the distinction is being made. To clarify:
The term Open Systems was used, primarily in the 1980's, to refer to systems that were hardware-and-software interoperable between different vendors and therefore avoided vendor lock-in (with varying degrees of success). Standards such as POSIX, SVID, and the Single Unix Specification provided source-code portability, and network standards such as TCP/IP provided network interoperability. However, Open Systems were still often proprietary, did not include source code, and were generally Unix-centric.
Open Source is software for which the source code is freely distributed (though the term is actually more formally defined). This software may be any type of program: an operating system, applications for an iSeries system, utilities for a Windows system, or games for a Mac. Since source code is provided, the software can be further enhanced and developed, and derivative works can be created.
There's not really much in common between the two. For good measure, one more definition:
Free Software is Open Source software protected by Copyleft licensing so that access to source code and freedom to use that code is preserved for recipients of derivative works. This is also formally defined.