https://www.gamasutra.com/blogs/Bric...g_in_Games.php
A Design Definition of Grinding
For a game designer, grinding can be defined as a part of the game that has both:
Incredibly strong Long Term Incentive to keep the player going forward
Base Mechanics and Punishment and Reward Systems that have already been mastered by the player
In a grind, the player wants to keep going. This is probably because they have already put a substantial amount of effort into the game, and they would like to see it through to the end. If a player has been planting veggies in FarmVille for weeks, and they are very close to being able to afford the barn, then they will be compelled to continue in order to make their previous effort worthwhile. This is an incredibly strong Long Term Incentive; a reward that will come to them in the future in exchange for action in the present.
But a powerful Long Term Incentive on its own doesn’t make it a grind. In addition the player must be performing the same actions over and over, actions that they have already mastered. Walking down a short hallway and opening a single door to find your friend isn’t a grind. Walking down the hallway for 30 minutes and then opening no less than 10 doors is a grind, because you will have already masted the activity long before you complete the challenge.
The new guardian blue prints are a great example of this. Took me about 5 or so hours to kit out the ship, get over there, collect enough mats, work out the puzzle and get a blue print. It was fun, challenging, atmospheric. But they can't stop there, this is elite. I have to do it another 7 times, for one weapon. Hours tacked on, that were not challenging, interesting.. or even atmospheric anymore. Just repetitive, walking down a hall way, repeatedly opening doors, to finally unlock the goal.