There are three types of piano keys: white, black, and gray. Each key has a different pitch or note when played individually. In the same way, if you want to play all the notes in an octave at once, there are several ways you can do it.
One way is to use three different keyboards with three keys each. This will allow you to play all 12 notes in an octave at once.
Another method is using two different pianos tuned to the same frequency (12 notes per octave). If you combine these two instruments together by having them share a common soundboard or action board, then this allows for playing all 12 notes in an octave at once.
Finally, if one instrument can produce more than one note within its range, then it is possible to play every note of the scale simultaneously by combining multiple instruments that produce that scale (ex: major and minor scales).
Hope this answer helps! Thanks for reading!
You could call it something like a “cluster-chromatic chord” but this is not particularly specific as to exactly what notes are being played, because if you pressed down 7 notes in a row on a piano, for example, that could also be called a cluster-chromatic chord.
I presume what you want is a description of the chord using jazz chord symbols. To answer this, I will first concentrate on only 1 octave of chromatic notes, from A to G#. Here goes.
Firstly, the bottom note of the piano is an A, so that is the root note of our chord. We want to describe as many notes as possible, with the least symbols. Off the top of my head, the best way to start here will be to say it’s A 13 (#11) chord. Generally, this means that you play the root, major 3rd, 5th, dominant 7th, 9th, sharp 11th (sharp 4th), and 13th (6th). So with A as our root, we have an A 13 (#11) chord, with the notes A, C#, E, G, B, D#, and F#.
These are 7 notes of the 12-note chromatic chord we want. The next step will be to add the sharp and flat 5ths and 9ths. We could express this by listing the alterations individually – i.e. A #11 (13) (b5) (#5) (b9) (#9) – however, an ‘alt’ chord generally means a chord with all the sharp and flat 5ths and 9ths. Since we want to talk about it using the least symbols possible we will call it A alt 13. This chord contains the notes A, C#, F, G, Bb, B, C, D#, F#. The #11 is now redundant as there is already a flattened 5th.
I will make it a major 9th chord as well, which turns the G into a G#, and also makes clear that the 9th and 5th are required, B and E. A maj9 alt 13.
There are now 10 notes in the chord; we are missing G and D. The G is the dominant 7th which we removed by using a major 7th. We can reapply it by using the rare symbol #13. This means the 13 in our chord must change to a 6, as you can’t have both 13 and #13. When there is both a 6 and a major 9 in a chord, it is generally expressed as maj6/9. A maj6/9 alt (#13).
Finally, the D can be added using add4. A maj6/9 add4 alt (#13).
This is a chord that would usually be voiced similar to the following: A, C#, E, F#, G#, B, Eb, F, G, Bb, C, D. Yes, it is spread over 3 octaves to try and space the notes out and make it sound nicer, but the chord still sounds pretty hideous.
However, one possible voicing of this would be A, Bb, B, C, C#, D, D#, E, F, F#, G, G#. This is also known as the chromatic scale. If you voiced it this way in every octave of the piano, plus the four extra notes at the top, you would be pressing down all 88 notes.
A maj6/9 add4 alt (#13). Wow.