What's a glossary term?

I'm hacking on some tool that indexes the localizable strings in our apps.

One of the fall-outs could be a glossary tool, i.e., which terms in Firefox, Thunderbird, etc should localizers bother to get consistently translated.

Which raises an interesting question, where do you draw the line? What's a good metric to use to define a glossary? Are there glossary-based applications that don't need a cut-off at all?

Insights welcome.