Nitrous, short for nitrous oxide (N2O) and sometimes abbreviated as nitro, is used to boost the speed and acceleration of a vehicle in Grand Theft Auto: San Andreas, The Ballad of Gay Tony and Grand Theft Auto V.
GTA San Andreas
Nitrous Oxide Systems (NOS; pronounced 'nos') are systems that can be installed on modifiable vehicles in the three available mod garages around the state. When installed on a vehicle, the "fire" button can be pressed to activate the NOS. The use of the NOS will increase the speed and acceleration dramatically, along with exaggerated motion blur and bright blue flames spewing out of the exhaust pipe(s). After a short time the nitrous exhaust ceases and the vehicle may revert back to its normal top speed. The boost is followed by a delay that won't allow a player to use another boost right away.
To know when you can fire a Nitro boost again, look at the exhaust after a depleted boost. If the fumes are colored blue, the boost is still in its "delay" state. If the fumes revert back to the normal, grey fumes, then the Nitro is ready to be fired again.
All mod garages have nitro as an available modification. Nitro comes in three "strengths": 2x, shown with a small tank; 5x, with a large tank; and 10x, with 2 small tanks. Each option determines how long you can use a Nitro boost.
Upon completion of the Taxi Missions, all Taxis have a 2X Nitro boost installed.
There is a secret of using nitrous all the time. Just bring the car to a full stop, exit it, and enter again. The nitrous is fully recharged.
NOTE: Nitrous Oxide Systems cannot be applied or activated on motorcycles!
The Ballad of Gay Tony
Nitrous is limited during races, although the boost can be controlled. Much like in Need for Speed and other similar games, a small bar next to the radar will show the amount of nitrous available, which will regenerate after some time driving at speed. Unlike in Grand Theft Auto: San Andreas, the nitrous in TBoGT can instantly catapult a vehicle to its top speed, or close to it. When Nitrous is ready, a high-pitched humming noise can be heard from the car, along with small purple to orange fumes from the exhaust pipes.
There is a trick in TBoGT Race and GTA Race mode; when the race is about to begin, hold the handbrake and the acceleration buttons at the same time until the timer says GO, then release the handbrake, and if done correctly, the car will get a HUGE boost of speed that will easily propel it into 1st place, and you will not waste any nitro the first time, but will the next time you use the nitro in that race. (this does NOT work when the Host of a GTA Race selects the APCs.) Also, unlike in Grand Theft Auto: San Andreas, motorcycles are also equipped with nitrous, but only in Race and GTA Race modes. This can be VERY dangerous, especially with traffic set to high. Hitting anything with nitrous equipped on a bike will severely injure or even kill the player on impact.
Grand Theft Auto V
Nitro is making yet another come back in GTA V, as first seen in the gameplay video at 3:22.