Grinding is a term used in video gaming to describe the process of engaging in repetitive tasks during video games.  The most common usage is in the context of MMORPGs like World of Warcraft, Tibia, or Lineage in which it is often necessary for a character to repeatedly kill AI-controlledmonsters, using basically the same strategy over again to advance their character level to be able to access newer content. MUDs, generally sharing much of the same gameplay as MMORPGs, encounter the same problem. Grinding may be required by some games to unlock additional features.
Synonyms for grinding include the figurative terms treadmilling (a comparison with exercise treadmills) and pushing the bar (it can be a reference to a weightlifter "pushing the bar" on a bench press, over and over to get muscle gains, or a reference to Skinner boxes in which animals, having learned that pushing a button will sometimes produce a treat, will devote time to pushing the bar over and over again, or also can be a graphical reference to push the character's experience bar to higher values). Related terms include farming (in which the repetition is undertaken in order to obtain items, relating the activity to tending a farm field), and catassing, which refers to extended or obsessive play sessions. Used as a noun, a grind (or treadmill) is a designed in-game aspect which requires the player to engage in grinding.
Grinding is a controversial subject among players. Many do not enjoy it, and disparage it as a symptom of poor or uninspired game design. Others embrace it, claiming that all games feature grinding to some extent, or claim to enjoy the practice of regular grinding. Some games, especially free to play games, allow players to bypass grinding by paying additional fees.
Video games are masterful in their ability to find the right difficulty level to engage their user. The aim is to create something that is challenging enough to generate some sense of accomplishment when objectives are met.