Source : Webster's Revised Unabridged Dictionary (1913)

Frictional \Fric"tion*al\, a.
   Relating to friction; moved by friction; produced by
   friction; as, frictional electricity.

   {Frictional gearing}, wheels which transmit motion by surface
      friction instead of teeth. The faces are sometimes made
      more or less V-shaped to increase or decrease friction, as
      required.

Source : WordNet®

frictional
     adj : pertaining to or worked or produced by friction; "frictional
           electricity"; "frictional heat"; "frictional gearing"