Railroading
The degree of determinism in a tabletop game. Railroading occurs when the GM has a particular story or sequence of events planned out and will not allow the players to deviate from it.
A small amount of railroading occurs in nearly all campaigns and is considered reasonable and necessary for a satisfying story. Excessive railroading, however, destroys the illusion of freedom that RPGs provide and alienates the players. For this reason it is important that a GM be able to improvise when his players go off the trail he's marked out for them without simply forcing them back onto it again.