I know of no such definition on AZB. Perhaps someone has posted something useful in the aiming subforum or perhaps no one has ever posted anything useful there.
But here is a working definition for now:
A "valid" aiming system is a set of instructions for setting up the line of a cue stick so that the cue ball will drive the object ball towards the pocket. The instructions must be complete enough that a computer could be programmed to execute the shot given that the computer has perfect vision, perfect knowledge of the positions of the balls, and perfect control of the cue stick.
A valid system is a good system if it satisfies the above and also will drive all object balls it's applied to into the center of the pocket when followed perfectly. There are lots of valid systems (they have clear instructions) that are very poor at pocketing balls.
For me, "move the stick around until it feels like the shot will go" is not a system. It is also the way most people play because they are people and not computers.
Very few systems include throw accurately enough to to be considered good. Ghost ball is mediocre unless you correct for throw. The amount of throw you get depends on whether you have draw/stun/follow on the cue ball. How many self-styled "systems" tell you to aim thinner for a stun shot? And how many include the fact that throw varies with speed?
A good example of a valid but bad system is the one Hal Mix describes in his book. The instructions are clear and the results (if the instructions are followed) are pitiful beyond belief. That system does get you to hit the ball on the correct side, but that's about it. Mix played well. He did not apply his system.