Many students struggle with the ideas of division by zero and the zeroth power. Is the value zero, or infinity; or is it undefined, or something else? Sometimes these are called undefined or indeterminate. What is that all about? See these pages from Ask Dr. Math:

Dividing by zero

n^0 (any number to zero power)

0^0 (zero to zero power)

Large numbers and infinity

And how about the square root of a negative number? Is that undefined, or "imaginary"?

Imaginary numbers