A keyboard instrument is any musical instrument played using a musical keyboard. The most common of these is the piano. Other widely used keyboard instruments include various types of organs as well as other mechanical, electromechanical and electronic instruments. In common language, it is mostly used to refer to keyboard-style synthesizers.
Among the earliest keyboard instruments are the pipe organ, hurdy gurdy, clavichord, and harpsichord. The organ is doubtless the oldest of these, appearing in the 3rd century BC, although this early instrument—called hydraulis--did not use a keyboard in the modern sense. From its invention until the 14th century, the organ remained the only keyboard instrument. Often, the organ did not feature a keyboard at all, rather buttons or large levers which were operated by a whole hand. Almost every keyboard until the 15th century had 7 naturals to each octave.
The clavichord and the harpsichord appeared during the 14th century, the clavichord probably being the earlier. The harpsichord and the clavichord were both very common until the widespread adoption of the piano in the 18th century, after which their popularity decreased. The piano was revolutionary because a pianist could vary the volume (or dynamics) of the sound by varying the vigor with which each key was struck. The piano's full name is "gravicèmbalo con piano e forte" meaning "harpsichord with soft and loud" but can be shortened to "piano-forte", which means "soft-loud" in Italian.
Keyboard instruments were further developed in the 20th century. Early electromechanical instruments, such as the Ondes Martenot, appeared early in the century.
Much effort has gone into finding an instrument which sounds like the piano but lacks its size and weight. The electric piano and electronic piano were early efforts that, while being useful instruments in their own right, were not successful in convincingly reproducing the timbre of the piano. Electric and electronic organs were developed during the same period.
Significant development of the synthesizer occurred in the 1960s and has continued ever since. The most notable early synthesizer is the Moog synthesizer, which used analog circuitry. In time, digital synthesis, using actual piano samples, has become common.