What is an undefined fraction?

Answer 1

Any fraction with zero for the denominator. This is because zero cannot be multiplied by any number to be anything but zero - you cannot divide by zero. This is why it is called "undefined."
The only exception to this rule is 0/0, which is called indeterminate, because technically any number can satisfy this expression. example:

a/b=c
which means that:
a=b*c

if b=0, and a is anything other than zero, then no matter what c is, b*c cannot equal a
5=0*c cannot be true no matter what.

but, if a is zero and b is zero, then c can be anything, and it still works, which is why 0/0 is indeterminate instead of undefined.