Originally Posted by [many people]:"Grinding" is just one of many, many bits of jargon appropriated by or invented within the MMORPG field.
What is "grinding"?
Grinding basically means doing the same thing over and over for long periods of time in order to gain some desirable thing, such as an epic piece of loot or enough experience points (XP) to allow your character to gain a level.
Wikipedia has a good entry on "grind," but I'll quote here from Wikipedia's entry for MMORPG:
In nearly all MMORPGs the development of the player's character is a primary goal. Many titles feature a character progression system in which players earn experience points for their actions and use those points to reach character "levels," which makes them better at whatever they do. Traditionally, combat with monsters and completing quests for NPCs, either alone or in groups, is the primary way to earn experience points. The accumulation of wealth (including combat-useful items) is also a way to progress in many titles, and again, this is traditionally best accomplished via combat. The cycle produced by these conditions, combat leading to new items allowing for more combat with no change in gameplay, is sometimes pejoratively referred to as the level treadmill, or "grinding." The role-playing game Progress Quest was created as a parody of this trend. [emphasis mine]In fairness to the other side of this question, some people seem to enjoy grinding. It doesn't take a lot of mental effort, and it's a safe way to make progress. So grinding is popular with some people who have lots of free time but little interest in learning complicated new features. The Terra Nova thread "In Praise of the Grind" explored some of these possibly positive aspects of grinding.