flare is basically the amount the ball wobbles down the lane. the track marks or oil marks on your ball are also called flare rings.  These rings indicate the surface of the ball touching the lane each time it wobbles.  More flare means more hook potential because more ball surface is touching the lane...fresh ball surface.

flare is in relation to the differential of the low rg and high rg of the ball. (rg=radiaus of gyration) ..the difference between the low and high is differential which determines the potential amount of flare.

amount of flare is in inches and is measured from the first flare ring (closest to fingers and thumb) ...to the last ring moving left (right handed bowler)

I beleive someone else can describe it better, and they may have a link that better describes it.
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JEFF
"A Mayhem and a couple of Hercs.......uhm.....Nice!.."