Continuing the discussion from How to write a series bible:
The ninth item on the checklist is “Special Terminology”
9 Special terminology. If you’re creating a sci-fi or fantasy series, you should list and describe any special terms or technologies that are key to understanding your show. Before Star Trek , there was no general understanding of terms like “transporters,” “warp drive,” “phasers” or “tricorders.” Pitching Game of Thrones required a cursory explanation of terms like “The Seven Kingdoms,” “direwolves,” “White Walkers,” and “warging.”
Here, I think, we are definitely past point of selling or pitching the series to the networks or studios or the campaign to prospective players, and into the stage of providing the writers and showrunners or the actual players with the information that they need to use to be able to contribute to the shared creation. This is the setting bible or campaign reference document.
A lot of GMs use RPG rule sets that have this stuff built in to the character representation rules and the fantasy shopping lists. Others buy commercial settings or use general-purpose RPGs to set their campaigns in well-known literary, cinematic, or TV universes. Not only are those easy to pitch, but they have the advantage of supplying this item, the vocabulary of special terminology specific to the setting. In many cases it is already familiar to many players through previous campaigns in the same setting or using the same games system, or through having read a classic book or watched a popular series.
Another way to reduce the load on the GM of producing this material, and on the character-players of studying it, is to adopt Ken Hite’s maxim. Start with the real world (present or historical) and add secret magic, wainscot fantasy, intrusion fantasy, emergent SF, an apocalypse, or other imaginative elements to taste.
If you do go all in on a fully fictional world (and I have to say that I usually do) then there is a narrow path to tread between not providing enough briefing that the character-players get any idea of what to do and supplying so much that they cannot or will not read or learn it.
With respect to special terminology specifically, there is a lot to be said for adopting a translation convention, even if it means using a term that is, strictly speaking or etymologically, a little bit off. For example, the people in my SF setting Flat Black are not actually speaking English but some future language descended through several centuries from the international lingua franca of the early 24th Century. And given the political sensitivities of the time at which the interstellar government agency was formed it is very unlikely that the founders would have been so brutally frank as to call it the semantic equivalent of “Empire”. They’d have used a euphemism. But “Imperator” and “Imperium” started out as euphemism, too. And by the time at which play is set it will have come to mean “empire”. So “empire” is the easiest thing to call the Empire.