“Cell phone,” or cellphone? “Home page,” or homepage? “Touch screen,” or touchscreen? Should such compounds be open, or closed? We see them both ways, so it’s difficult to know how to treat them — unless you use one simple test: Choose the form based on the context.
The natural progression for styling compound words is open to closed, often (but not always) with a hyphenated form as an interim phase. No ruling body authorizes the transformation, and no pattern or logic regarding the time frame applies from one compound to another. Some compounds stubbornly resist closing (“real estate”) or cling to their hyphens (mind-set), but closure is almost invariably inevitable.
Thus, for example, “sea water” at some arbitrary point transformed into seawater, with a transitional period in which both forms were commonly used, followed by preponderant use of the new form (though the old form nearly always persists to some extent).
Technological terms are a special case, for various reasons, including that they are coined by technologically minded people, who are not necessarily concerned about adherence to grammatical norms, and that, in the case of programming vocabulary, the practical issue of having a single string of characters to enter into a program is integral. Therefore, compounds referring to technological devices and procedures are likely to begin life as closed compounds or to be adopted in technological contexts in closed form.
And that’s the key to knowing how to treat them: In general-purpose publications, you’ll likely see “file name” and “screen saver” and “voice mail,” whereas in high-tech periodicals and on high-tech websites, you’ll probably find filename and screensaver and voicemail. (There are exceptions of course; note that on this site, I have reluctantly adopted website in place of “Web site,” and I have always preferred email to e-mail.) Consider your audience, and style technological terms as appropriate. And when in doubt, depending on the context in which you are writing or editing, consult mainstream or specialized publications for models.