Sticky is an industry term that equates to a product's ability to retain customers for a long period of time (years).

If you asked a seasoned game designer about what it would take to make a game sticky, you would most likely hear that it is important to promote bonding with other players through shared experience. Indeed, this is what makes a massively multiplayer game different from other genres, and therefore the game design should cater to bringing people together at all levels.