Pronunciation: (ob-vûr'zhun, -shun), [key]
1. an act or instance of obverting.
2. something that is obverted.
3. Logic.a form of inference in which a negative proposition is obtained from an affirmative, or vice versa, as “None of us is immortal” is obtained by obversion from “All of us are mortal.”
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.