The Collaborative International Dictionary of English v.0.48:

Convertend \Con`ver*tend"\, n. [L. convertenus to be converted.]
   (Logic)
   Any proposition which is subject to the process of
   conversion; -- so called in its relation to itself as
   converted, after which process it is termed the converse. See
   Converse, n. (Logic).
   [1913 Webster]