Well to me ideal clarity = clear without being overly bright/sibilant and no muddiness in any part of the frequency response. I should be able to hear any part of the music clearly be it the voice of the singer, the instruments or the rumbling of the bass whenever I feel like I want to focus my listening on any of them at any time. Being NOT ideally clear is when I wish to catch one part of the music be it the vocal, any instrument or the bass by focusing my attention to it, but I can never hear it properly/clearly because some other instrument/voice is masking the sound over it. The closest analogy I can think of is like color bleeding in pictures/photos. The edges are not clearly-defined due to color bleeding. You can't tell where one part ends and the other part begins. Of course, I think this ideal clarity is near impossible because there is always at least one component that is at the foreground at a given part of the music which may mask out the other sounds in the background, but I should still be able to pick out clearly what sounds are in the background. I am not sure if I make much sense here, but I feel a good IEM should at least try to achieve this level of clarity and to me the best IEM for my taste would be one that is as clear as I have described and while still retaining its overall richness/warmth or "synergy" if one wants to put it that way, and not becoming too analytical and dry like most BA-driven IEMs. That is why I prefer hybrids over those pure BA-type IEMs which are more suited for stage-monitoring and have a danger of being too analytical and placing a strain on the listener's ears instead of actually letting him enjoy the music.
I meant I asked the other poster because he sai clarity was important to him. But since you brought it up that definition wouldn't do much good in clearing up any definitional issues. You used the word being defined in the definition for one
Some people will see it as high resolution, ie not smearing of or missing many details. Others will see it as the crispness brought on by good dynamics. Someone else may see it as a lack of blackness in the soundstage, or to say that there is an openness or light around the instruments, not to be confused with a headphone being dark (which sometimes also has its own problems). Then there are those that associate it with brightness as you said. I'm sure there are other definitions still and combinations of definitions.
It's not exactly a rule imo. It's an unfortunate circumstance of subjectivity and a failure to effectively communicate.
Like I have described my definition of clarity above, I think it would match the 1st definition as mentioned by vwinter here, i.e. high resolution and no smearing of details.
I wish I can afford a collection of IEMs as huge as yours... Then I can compare them myself and don't need to scour the forums for reviews.