I'm doing an independent research in aeronautics (realistically I'm just learning the material on my own and testing out of it due to unforeseen circumstances last semester). One area of study that my professor advised me to research in depth was just this simple phrase "compressibility is abridged." It sounds simple- the word abridged means "to shorten." However I haven't the slightest clue what context it relates to. I do know it pertains to thin airfoil theory (depending on how good this forum is, I may post questions about that in the near future). I've searched the entire internet and the book and I feel like I can't find anything pertaining to this idea. Thank you in advance for any aid; and I really hope this particular proff isn't affiliated with this forum- that would be embarrassing.