- Edited
A question that one might ask or eventually ask is - is a digital piano a piano? Preview answer is yes. Absolutely. It is a piano. A real piano.
Deciding to start this thread here too. It is about piano, how it was mentioned that - back in the old/older days, there was/were no other method or technology to achieve 'piano forte', independent and 'adequate' soft-loud control of struck string sounds such as from a harp structure (enclosed or semi-enclosed by a cabinet) using a harpsichord style of keys for the input.
So - achieving piano-forte (shortened to 'piano') could only be done with the full mechanical method. Adequate soft-loud control could eventually be achieved with acoustic pianos, which are/were the first type(s) of piano.
It is later - when the technology allowed it - that other types of pianos could be realised. This includes electric pianos and digital pianos.
If the sound isn't limited to 'struck string', then it is absolutely possible to include electric pianos as pianos. After-all, the clue, which sits right in front of our noses and eyes - is the word 'piano', which is seen in the words - electric piano, acoustic piano, digital piano, hybrid piano (where hybrid pianos are actually digital pianos - but just needed different wording to distinguish the sort of key mechanism).
And in a recent thread - somebody mentioned semi-weighted keys (aka spring or springy keys) - aka what they call 'keyboards' or synth-type keyboards/mechanisms. In my own opinion - regardless of whether those instruments are outputting struck-string sound (ie. acoustic piano sort of sounds) or not, even synth keyboards are in the piano category.
They all go under the 'umbrella' of piano -- which include acoustic piano, synth keyboard, electric piano, digital piano, hybrid piano. All pianos. All real pianos. To distinguish - they call them by their type - eg. acoustic piano, digital piano etc.
And they are all musically excellent and amazing.
Piano forte - adequate soft loud control of the sounds with multi-note input mechanisms - is the essence of 'piano' - or idea of piano. And the additions added - eg. sustain, una corda etc are extras.
Analogy is planes and cars etc. Originally - the full mechanical method (plus a little bit of electricity of course, for the spark needed for combustion). Later, other types/kinds. And they are still planes and cars in terms of the particular function when they're doing that particular function.