Your definition of key seems very bound by www.dictionary.com which seems rigid. Step outside, Mr. A, and into the grey. No need to dwell in such a world of black and white.
A key to me and many (and correct me fellow boardies if this is wrong), is any comic that is desired above others for any reason. Art, story, character, cover...whatever. It could be a key first appearance, a key moment in a character's history, a key cover, a key story by a notable author. If it is desired more compared to other issues that preceed it or follow it for some reason and tends to always be missing as one flips through a longbox (Logan's run 4, 5 annnnd 7...drat) then it has some sort of key status.