Source : Webster's Revised Unabridged Dictionary (1913)

Tesla coil \Tes"la coil\, Tesla transformer \Tes"la
trans*form"er\ [After N. Tesla, American electrician.] (Elec.)
   A transformer without iron, for high frequency alternating or
   oscillating currents; an oscillation transformer.

Source : WordNet®

Tesla coil
     n : a step-up transformer with an air core; used to produce high
         voltages at high frequencies