can be defined in many ways depending on the situation...for squirrel hunting with a .22 it's probably an inch at 25 yds...for tin cans on a sunny Saturday, a cpl inches is OK...for SD use, night or day, from any position, it's something else...I attended the Defensive Handgun Course described below and here's what they train for.
For concealed SD carry with any handgun, Front Sight, the NV based shooting school defines successful accuracy as a two shot group that you can cover with the palm and fingers of your hand....from any distance...to the thoracic region ... or a single shot delivered to the cranial region. See the pic below for the black line defined cranial and thoracic regions.
They teach that a group smaller than your hand equates to lost time that you can ill afford in a SD situation. A group larger than that means that you need to slow down a bit to ensure hits when it counts.
The reason for the hand sized group in practice is that your accuracy degrades by at least 50% in a SD situation and that the resulting shot placement will be twice as large...so if you double your practice hand size shot group it'll still be in the thoracic region, when the chips are down.
Front Sight uses SD exercises, most all of them 'controlled pairs', shot from 3 to 15 yds in their Defensive Handgun four day course to achieve that level of proficiency. A part of the qualification testing on the last day will see a successful student place two shots to the thoracic region from the holster in 1.8 seconds shot from the 5 yd line. Time limits are increased for greater distances, but even from 15, the limit is only 2.5 seconds. Their four day course requires 600 rounds to complete...so there's plenty of practice to get that required level of hand eye coordination and muscle memory...and all of it is from the holster.
At any distance...drawing/presenting your handgun, then firing as fast as you can acquire the sights, and placing all fired shots under that palm sized group is the goal of training. Shots must be placed in the the thoracic region...see the attached target.
For adversaries that are not stopped by the two shots fired in the above manner, a third fight-finishing shot is fired to the cranial region...note the small area defined on the target...
A visit to their web site will give you a better idea of the reasons behind their advocacy of these standards. While they train primarily with auto-loaders of most any make, the accuracy standards are the same for those training with a revolver.
Best Regards, Rod