Oh, apparently baseball people use "length-to-weight ratio" to describe a bat, as I learned from the people who talk too much before the Saturday afternoon game on Fox today. This is calculated by taking the weight (in ounces) minus the length (in inches), and in the major leagues can't be less than -3.5.
Of course, it's actually a difference, not a ratio.
It looks like some people call it the "differential", though, which is fine with me -- to me "differential" has other connotations, but expecting mathematical terminology not to collide with terminology used in other things is a Bad Idea. (Although why not just call it the "difference"?)