Despite the fact that it’s been slow to appear in traditional dictionaries, the adjective “standalone”—meaning “independent”—has become hugely popular in recent years. There are standalone electronic devices, standalone computer applications, and standalone businesses. Authors known mainly for writing books in a series who decide to write a single work unconnected with any series are said to have written a standalone novel.

You’re more likely to find what you’re looking for in dictionaries under the hyphenated spelling “stand-alone.” Formal edited English still usually prefers this version. There is a strong tendency for such hyphenated forms as “on-line” to get smooshed together into one-word spellings (for instance, “online” is now standard as an adjective). That process is clearly happening with “stand-alone,” but it’s safer to use the hyphen unless you know for sure that the audience you are writing for prefers the unhyphenated form: write “stand-alone device,” etc.

Rendering this adjectival form as two unhyphenated words (“a stand alone device”) is just a mistake.