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  • You may want to clarify the first part of your answer: Bruce's article already explains why one should not use a constant epsilon (such as the ones defined in a header file) for tolerance comparison. Also, in many cases, an error of a few millions of ULPs is not something to worry about, because in most applications, we care more about the significant digits more than the errors in the least-significant digits, because double-precision already gives many more digits than we care about. Commented Aug 10, 2013 at 23:08
  • @rwong - as I read it, the question was to identify the correct term to use for a constant's name. So that's why I provided the float.h reference along with a few others to machine epsilon. The article from Dawson is something I found while searching for the IEEE 754 reference and I thought was relevant to the OP's simplest formula for comparison. Many use that approach as a first attempt, and I included Dawson's article because it really goes into the nuances of just how tricky the comparison is. So I tried to directly answer the question and then point out why not to use it that way. Commented Aug 11, 2013 at 13:40