I advise very strongly against this.
First, if you give the name risk to P90 you are using a very general name, already in broad use in the statistical sciences, for a very specific term.
What then do you call "risk"? You are flying against a mighty wind: it is already a common word with a relevant meaning. People already understand risk broadly even if they are not well informed quantitatively: for example, people may get the relative risks of being shot by small children compared with being shot by terrorists the wrong way round.
Second, this seems too pessimistic. Many people understand 10, 50, 90%, so use those ideas directly. Showing a graph and using a simple numeric example to illustrate are obvious possibilities.
Third, but also important, better informed people will just be puzzled by your not using standard terms. I am an example: I use meteorological and climatological data and am not a statistician, but I understand mean and quantiles.
Whatever you do, some people won't understand, but inventing non-standard terminology (or adopting new meanings for standard terms or common words) isn't the way out.