The Collaborative International Dictionary of English v.0.48:

Electrotonus \E`lec*trot"o*nus\, n. [NL., fr. combining form
   electro- + Gr. ? tension.] (Physiol.)
   The modified condition of a nerve, when a constant current of
   electricity passes through any part of it. See
   Anelectrotonus, and Catelectrotonus.
   [1913 Webster]