Danny and Trevor share their top 5 lists of Game Expansions. To qualify as an expansion for their lists the game cannot be a stand-alone expansion. What they mean by that is the expansion must be dependent upon the base game to be a true expansion. If the expansion can be played without the base game it is not a pure expansion in their minds.
Board Game Geek defines a game expansion as additional equipment for a game, usually sold separately. Expansions can be used to add a variant or add additional scenario, to add more players for the game, add new maps or tracks for a game, etc.
Typically an expansion is exactly that… it expands the existing game. It may add the ability to have more players, insert more scenarios, give players more equipment, etc. Expansions keep popular board games relevant after players have gone through many iterations of game play. It helps to keep your board game investment fresh for a longer period of time.
If you own any of the base board games mentioned in these lists, you should give the expansion a try to extend the life of your favorite game.