A quality of a
video game--the degree of enjoyment a player can derive from playing a game after beating it. It is essentially irrelevant to games which are intended to be played for reasons independent of
accomplishment,
exploration, or
story (such as most sports and fighting games)--it can be essential to those which rely on these elements.
To illustrate by example,
Metal Gear Solid is almost utterly devoid of replay value--if you play the game a second time, you will experience essentially the same events in the same order--though your tactics may vary slightly, your objectives don't change. That said, it was still a fantastic game while it lasted, but low replay value means that, once the game is finished, there is no reason to keep it. By contrast,
Diablo has high replay value; each time you play, the map is recreated entirely differently, and the objects you gain may vary significantly, so if you enjoyed playing it long enough to beat it, you'll probably enjoy it again. Also, it is possible to choose different
classes, which have very different
play styles, and play through as each of them.
The Sims and other simulation games may have transcended replay value altogether by failing to have any sort of goal--you can just play until you stop wanting to; there will always be more to do.
Replay value is commonly increased by adding
secret areas or other
secrets which are not necessary to objective completion, de-linearizing the
plot or
gameplay, allowing the player to begin the game as a different character or sort of character, and allowing the player to play through essentially the same game with certain minor, but important, differences.