fudge factor
A value or parameter that is varied in an ad hoc way to produce the desired result. The terms "tolerance" and slop are also used, though these usually indicate a one-sided leeway, such as a buffer that is made larger than necessary because one isn't sure exactly how large it needs to be, and it is better to waste a little space than to lose completely for not having enough. A fudge factor, on the other hand, can often be tweaked in more than one direction. A good example is the "fuzz" typically allowed in floating-point calculations: two numbers being compared for equality must be allowed to differ by a small amount; if that amount is too small, a computation may never terminate, while if it is too large, results will be needlessly inaccurate. Fudge factors are frequently adjusted incorrectly by programmers who don't fully understand their import.