An electronic organ is an electronic keyboard

Electronic keyboard

An electronic keyboard is an electronic or digital keyboard instrument.The major components of a typical modern electronic keyboard are:...

 instrument which was derived from the harmonium

Harmonium

A harmonium is a free-standing keyboard instrument similar to a reed organ. Sound is produced by air being blown through sets of free reeds, resulting in a sound similar to that of an accordion...

, pipe organ

Pipe organ

The pipe organ is a musical instrument that produces sound by driving pressurized air through pipes selected via a keyboard. Because each organ pipe produces a single pitch, the pipes are provided in sets called ranks, each of which has a common timbre and volume throughout the keyboard compass...

 and theatre organ

Theatre organ

A theatre organ is a pipe organ originally designed specifically for imitation of an orchestra. New designs have tended to be around some of the sounds and blends unique to the instrument itself....

. Originally, it was designed to imitate the sound of pipe organs, theatre organs, band sounds, or orchestral sounds.

Today, it has developed into three or more types of instruments;

• the Hammond organ
  Hammond organ
  The Hammond organ is an electric organ invented by Laurens Hammond in 1934 and manufactured by the Hammond Organ Company. While the Hammond organ was originally sold to churches as a lower-cost alternative to the wind-driven pipe organ, in the 1960s and 1970s it became a standard keyboard...
  
  -style instruments used in popular music genres;
• the digital church organs, which imitates a pipe organ and is used primarily in churches;
• and various other types of organs including combo organ
  Combo organ
  A combo organ is an electronic organ of the frequency divider type, generally produced between the early 1960s and the late 1970s. The combo organ concept, at least in the context of mass-production, is thought to have been born from the accordion, probably in Italy, as the brainchild of necessity...
  
  s, home organs, software organs, etc.

Harmonium
The immediate predecessor of the electronic organ was the harmonium

Harmonium

A harmonium is a free-standing keyboard instrument similar to a reed organ. Sound is produced by air being blown through sets of free reeds, resulting in a sound similar to that of an accordion...

, or reed organ

Reed organ

A reed organ, also called a parlor organ, pump organ, cabinet organ, cottage organ, is an organ that generates its sounds using free metal reeds...

, an instrument that was very popular in homes and small churches in the late nineteenth and early twentieth centuries.