The sequential order of events or levels. Linearity is directly related to interactivity. A movie is linear, it has a begining a middle and an end. Generally a movie is only viewed in it's linear order. This makes it non-interactive. In most games a story is intended and some linearity is needed. For example, in StarCraft you can't play the protoss levels unless you have beaten the human levels. This allows plot to be added to the game. (See Also Non-linearity)
In computer and video games, linearity denotes that the objectives of the game must be completed in a fixed sequence, sometimes revealing elements of the plot, in order. Non-linearity means that the player has a selection of options. Gameplay typically combines both linear and non-linear moments, but whole games may be characterized as being mostly linear or non-linear.