RG - Radius of Gyration

The measurement of the effective weight distribution in a ball as it relates to the moment of inertia. It essentially is an indication of the resistance to rotation motion (how easily the ball rolls or does not roll). It is equal to the square root of the moment of inertia divided by the weight. Therefore, different weights have a slightly different RG values. However, when in the upper weights of bowling balls, the values are so small they virtually do not come into play.

Length

Length is an evaluation of how far a ball will travel before it begins to hook. The higher the number, the further down the lane it goes before it hooks. Length does not include skid produced by lane conditioner, additional fine sanding, or the use of "liquid sandpaper" polishes. These values are based upon stock finishes and the overall cover of the bowling ball.

Weight Block

The inner portion of a bowling ball that influences ball reaction, based on its density and position. This is where layouts or drillings come into play.
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Track Flare

Track flare is the migration of the ball track from the bowler's initial axis (the axis upon release) to the final axis (the axis at the moment of impact with the pins), thus a new ball surface touches the lane upon every revolution. Increased track flare gains greater rotational energy and hitting power. Decreased track flare creates greater ball skid. No track flare will have a ball rolling on an oily surface each revolution.