Why not instead use real definitions when they make sense, i.e. "invisible" means you're transparent and can't be seen but can still be heard smelled etc., and when a term is needed for something game-related either use a different word or - better yet - make up something new. We all know what "hit points" are, for example specifically because it's a bespoke term created for the specific purpose of referring to what it refers to and nothing else.
Here, the term "unseen" could be used as a term for someone who can't be seen due to cover or hiding-in-shadows or lack of light whatever but who is not actually invisible. The spell that already uses that term would need to have its name changed to Invisible Servant, of course, but whatever.