Grind is an activity that require repetitive activity with no real variation for an extended period of time in order to gain something in game. Mining is a grind (especially with non AOE tools) as you hit rock over and over again in order to gain enough resources to actually craft or build. If the entire time I have to play is only repetitive activities then it is uninteresting and grind.
I think it is interesting that I read about players that are watching videos and listening to music while doing certain things in Boundless like mining. If the activity is not engaging enough for you to want to focus your attention on it then does it belong in a game that is supposed to be fun?