Strictly speaking an Electrophone is a term used to describe instruments that involve electricity.
The category was added to the Hornbostel Sachs musical instrument classification system by Sachs in 1940.
Electrophones are broken into 3 sub-categories: Electrically actuated acoustic instruments
Electrically amplified acoustic instruments
Instruments which make sound primarily by way of electrically driven oscillators, such as theremins or synthesizers, (radioelectric instruments).
The original classification has been modified somewhat in recent times and now refers only to musical instruments which produce sound primarily by electrical means.