Have you heard the expression "the daily grind"?
It usually refers to your full time job. It is what you do each day every day, with time off for good behaviour, of course.
The same applies to a 'grinder' in poker. Do it pretty much every day and treat it almost like a second job. Have some sort of 'work ethic' and not just dip in and out a couple of times a week.
Although I agree that most grinders would multi-table I do not think that is necessary in order to be called a grinder. Someone who plays a particular MTT every day for 5-6 hours could equally be a 'grinder' IMNSHO, provided they did so with the intention of winning, or trying to.
The opposite, apart from not playing at all, would be a recreational player. A recreational player might play in the same MTT as above but only a couple of times a week and doesn't really mind if they don't get ITM, they are just passing the time or maybe hoping to get to meet a buddy or two for some banter.