Source : Webster's Revised Unabridged Dictionary (1913)

Spark gap \Spark gap\ (Elec.)
   The space filled with air or other dielectric between high
   potential terminals (as of an electrostatic machine,
   induction coil, or condenser), through which the discharge
   passes; the air gap of a jump spark.

Source : WordNet®

spark gap
     n 1: a component of an ignition system; consists of two shaped
          electrodes and the space between them
     2: the gap between two high-potential terminals