The Collaborative International Dictionary of English v.0.48:

Syntonic \Syn*ton"ic\, a. (Physics)
   Of or pert. to syntony; specif., designating, or pert. to, a
   system of wireless telegraphy in which the transmitting and
   receiving apparatus are in syntony with, and only with, one
   another. -- Syn*ton"ic*al, a. -- Syn*ton"ic*al*ly, adv.
   [Webster 1913 Suppl.]