Accuracy is a weapon statistic found in all weapons except Shotguns, Melee weapons, Explosives and certain Specialist items. It describes how close to the center of the crosshair the weapon's projectile is expected to hit.
A weapon with high accuracy will shoot very close to the center of the crosshair while a weapon with low accuracy could have a bullet fly very far away from it. This statistic goes along with Recoil, since accuracy decreases as recoil increases.
A high accuracy is very common among Sniper Rifles, especially slow firing sniper rifles which have an accuracy of nearly 100. For a long time the M24 Family contained the only weapons with this perfect accuracy. The DSR-1 Tactical was the first one hit kill sniper rifle released with perfect accuracy. The M99 Bigshot was the second to share this trait.
Rocket launchers have a special role in Combat Arms. Since they fire in a straight line, it can be assumed that they have an accuracy of 100 as well.