Bemoptil23, this question is actually a very, very good one, unfortunately there is no real right answer or explanation of the definition, our perception of "grainy" can vary as wide as our tastes.
I perceive grain as a fragmentation of sound within a sound, almost as if there were little minute stops in the sound where the frequency that the graininess was, it seems that the more frequency it is perceived in, the larger the grain.
I have perceived amps with total grain and to me, it sounds like the sound stops and starts but so fast that it's almost imperceptible.
This is of course will be one opinion in many but I hope I was of some help.
I perceive grain as a fragmentation of sound within a sound, almost as if there were little minute stops in the sound where the frequency that the graininess was, it seems that the more frequency it is perceived in, the larger the grain.
I have perceived amps with total grain and to me, it sounds like the sound stops and starts but so fast that it's almost imperceptible.
This is of course will be one opinion in many but I hope I was of some help.