I hate to beat this to death but here is the defination out of my Sierra manual:

"Point blank range is a concept that is very important to hunter's and silhouette target competitor's. The point blank range of any gun is the range distence out to which a shooter can hold right on his game or target. In other word's, the shooter does not have to hold high or low to correct for the bullet trajectory.........................As long as the range to a game animal is not farther than the point blank range of the gun, the hunter can aim at the center of the vital zone on the animal and be assured of a hit.............."

There is more on it in the manual, Eddition V 2nd Printing.