In last week's column I asked readers to respond to Microsoft's claims that Windows is the most open standard because users have voted with their pocketbooks. Surprisingly, just as many readers wrote in saying that they agreed with Microsoft. Or for that matter, who is the judge and jury.
Ubiquity means users are open to it.
Ergo, Windows is open. I don't know if there is a formal definition of open, but it has generally come to mean something that's not proprietary. Open means that no one party has control over the standard and that anyone can contribute to it. This keeps one company from having too much control over a specification and allows many different vendors to implement it without having to reverse engineer a spec. Microsoft, obviously, sees things a bit differently.