Given a chord, I can play its notes in random order (usually fast) for full note's duration. It's not an arpeggio, because arpeggios have ascending or descending order; and arpeggio ends when all its notes have been played. It's not a tremolo, because tremolo is repetitions of a fixed note/chord or alternating of two fixed notes/chords.
So, what is it called and how to notate it? It's very strange if there is no standard way, because I hear such things very often, especially in piano improvisations (it's a good choice for a long final chord, for example).
For myself, I notate it like this:
but I don't know how to notate it in the right way.