The Collaborative International Dictionary of English v.0.48:

Permistion \Per*mis"tion\ (p[~e]r*m[i^]s"ch[u^]n), n. [L.
   permistio, permixtio, fr. permiscere, permistum, and
   permixtum. See Permiscible.]
   The act of mixing; the state of being mingled; mixture.
   [Written also permixtion.]
   [1913 Webster]