Originally found in the first Tony Hawk's Pro Skater game, Gaps are named extra points players get for performing jumping over obstacles or performing a trick on a certain object. They would later be adapted by many skateboarding, snowboarding, BMXing and other extreme sports games.
In the THPS series (and many other sports games released by Activision), gaps show up in the trick string on the bottom of the screen as blue text. Point amount is based on how difficult the gap is and points for that gap decrease when the it is repeated. While most gaps will require the player to simply jump over an obstacle (i.e.: jumping over a rail) or across a gap (i.e.: jumping from one quarter-pipe to another one), certain gaps require a more specific action (i.e.: THPS2's School II level has 3 'Roll Call!' rails. These were 3 extremely long rails that requires the player to grind from one end of the rail to the other).