In math, what is an undefined fraction?

Rule 1: Division by Zero in a Simple Fraction. The denominator of any fraction cannot have the value zero. If the denominator of a fraction is zero, the expression is not a legal fraction because it's overall value is undefined. read more

I will assume that, by fraction, you mean rational number. A rational number is just a ratio of integers, as in mn, where m and n are integers. In that sense, there is only one restriction, which is that n≠0. So the only obvious undefined fraction in that sense would be ones with 0 in the denominator. read more