Unique Stunt Jumps
Each unique jump is a takeoff point leading to a landing area. Miss either and you do not get the jump. If your vehicle hits the takeoff point in more-or-less the right direction, the game usually switches to a slow-motion display of your jump. Then on landing you either get a message about success, or indication that you did not succeed.
Some jumps need high speed, thus require a motorcycle and sometimes the. In other jumps too much speed may lead you to overshoot the landing area.
Early GTA games gave no indication to distinguish unique stunt jumps you failed to clear from unique stunt jumps you have not found.
Users have developed add-on tools for these games such as Demarest's Uniquecall to list or show on the map which jumps remain.
Grand Theft Auto: Liberty City Stories shows each jump you've attempted on its map when you choose "Show Discovered Extras"