Meaning of contraposition

con•tra•po•si•tion

Pronunciation: (kon"tru-pu-zish'un), [key]
— n.
  1. placement opposite or against.
  2. opposition, contrast, or antithesis.
  3. the inference drawn from a proposition by negating its terms and changing their order, as by inferring “Not B implies not A” from “A implies B.”
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.