The Collaborative International Dictionary of English v.0.48:

Electro- \E*lec"tro-\ [L. electrum amber. See Electric.]
   A prefix or combining form signifying pertaining to
   electricity, produced by electricity, producing or employing
   electricity, etc.; as, electro-negative; electro-dynamic;
   electro-magnet.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Electro \E*lec"tro\, n.
   An electrotype.
   [1913 Webster]