It’s kind of like suspension of disbelief. Gameplay becomes grind ultimately when the player believes the tasks are artificial and arbitrary rather than naturally incorporated into what the character should be doing.
It is definitely one of the hardest MMO terms to define. Almost like pornography. You just know it when you see it (SFW).
It means that we’re just dolls.
p.s. I had an epic “grind” post – that spawned from the comments of Hobby Elements – covering when grind was used to teach, used to make a worthwhile feeling of accomplishment, etc… and I think the only intelligent thing that came out of the entire multi-rewritten post is what is above.