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Attributed to John Horton Conway. From unless, by analogy with the formation of iff from if.



  1. (mathematics, logic) Precisely unless.
    • 1990, James Glimm, The Legacy of John Von Neumann, American Mathematical Society, →ISBN, page 279,
      Partial Order: GH unlesss (unless and only unless) H ≥ some GR or some HLG.
    • 1999, V. K. Balachandran, Topological Algebras, 2000 North-Holland edition, →ISBN, pages 78–79 [1]:
      A subset   is called absorbing if to each   there is a real number   such that   for all   with  . Trivially the set   is absorbing; on the other hand   can never be absorbing (unlesss  ).
    • 2004, William Fraser, Susan Hirshberg, and David Wolfe, "The Structure of the Distributive Lattice of Games Born by Day n", in Integers: Electronic Journal of Combinatorial Number Theory 5(2) (2005), page 2,
      GH     unlesss HGR or HLG for some GRGR or some HLHL. ¶ (Analogous to “iff”, the term “unlesss” means “unless and only unless”.)