When people hear words like "movie," "song" or "TV show," they can usually agree on just what these concepts mean without too much trouble. Though specific instances within each medium may vary wildly from each other, pop culture has at least progressed to such a stage where we have a workable, overarching definition for each medium. For example, Arrival of a Train at La Ciotat could not be more dissimilar from Transformers, but everyone agrees that both are movies. Even John Cale's 4'33" counts as a song, but no one would want to put it on their iPod.
I originally wanted to begin this post by offering a workable definition of "videogame" in order to establish a framework for the medium equivalent to the ones above. However, after reading posts by others also attempting to come up with a definition (specifically Man Bytes Blog and The Quixotic Engineer), it became clear that this was a topic far too broad and complex to reach a consensus right now.
After reading Corvus's entry on Man Bytes Blog, it became clear to me that if we are to attempt to come up with a definition of "videogame" that excludes things like board games and tabletop RPGs, the main goal is to discover the essential element of videogames that are not present in any other type of game. But what element is this? A video screen? Nope, there are games that do without. Perhaps the presence of some type of electronic component? If so, then why isn't Electronic Battleship sold at GameStop? Clearly, videogames have become such a diverse medium that we are still struggling to even come up with a universal explanation of what they are. For more in-depth looks at this topic, I encourage you to read the blogs posted above.
I do feel, however, that there is a question about that I can answer: What is a videogame's primary purpose? That is to say, what can be derived from the experience of playing a game that cannot be gained from, say, watching a movie or listening to music? To put it plainly, the primary, distinct purpose of a videogame is to provide immersive interactivity.
When I say interactivity, I mean that the game must depend upon some type of action or activity from one or more players to start, progress and finish the game. Defining immersive is a bit trickier, but what it means in relating to interactivity is that a game must give the player a vicarious role or experience separate from the real world. The game must let players feel as though they are inhabiting a different person, or at least taking on a role different from their current experience. This could be anything from a fighter pilot, rock star, or an invisible being that arranges blocks that fall from the sky.
I say separate from the real world only to denote that, once a videogame is turned off, everything disappears. In chess, the pieces remain after one player reaches checkmate. In Dungeons & Dragons, the 20-sided dice remain long after the last kobold encountered by the party lies dead. Of course, the computer data that makes up the game itself does not disappear, and the DVD or cartridge housing it doesn't either. However, I don't count these as being part of the game for the same reason I don't count the box and instruction manual.
I believe immersive interactivity fits both as the definition of what games have offered since their beginning, and what we can expect them to offer for the foreseeable future. Every game I have played in my life (from "Super Mario Bros." to "Dance Dance Revolution," from "Pong" to "Portal") has depended on a human player to manipulate a character or object toward some objective. Also, they all let you experience something that would not be normally available in everyday life, or they present an everyday life scenario in which the player can experiment without fear of real-life consequences. "The Sims 2" lets you do a lot of the same things you can do in reality, but purposefully making your in-game character's life a living hell doesn't necessarily impact your own life.
I freely admit that the concept of immersive interactivity is, at this point, incomplete and insufficient. However, just as the debate over the definition of videogame carries on, so too will I work to refine what I mean by immersive interactivity. But no matter how much the elements of videogames (like graphics, controllers, and interfaces) change, I see no reason to expect immersive interactivity to become inherently different or less central to a game's make-up. Players are always going to be entering a separate, digital world when they load up their favorite game, and when videogames stop offering such experiences, then I assert that it will be time to come up with a term other than "videogame."