A negative number is a number less than zero. I don't know what a subtractive number is, so if it means something it is very uncommon.

I know it as a "negative number," and have never heard "subtractive number." Something that has changed in my lifetime is that numbers like -4 are now spoken aloud using the word "negative," e.g. "negative four." This is a good thing, because the usual way to read the subtraction symbol is with the word "minus." Thus, "10 - (-4) = 14" is read as "Ten minus negative four equals fourteen." In the old days, negative numbers were spoken using the word "minus," so that would have been spoken as "ten minus minus-four equals 14," which was very confusing. From a web search, I find that in describing the structure of Roman numerals, the phrase "subtractive notation" is used to describe the system in which 9 is written as IX rather than VIIII.