What is an isotope?

Isotopes are any of the different forms of an element each having different atomic mass.

Check

http://en.wikipedia.org/wiki/Isotope

"Isotopes are any of the different forms of an element each having different atomic mass (mass number). Isotopes of an element have nuclei with the same number of protons (the same atomic number) but different numbers of neutrons. Therefore, isotopes have different mass numbers, which give the total number of nucleons—the number of protons plus neutrons."

So basically for an atom like carbon, it has to have 7 protons (atomic number=7), but it can have a different number of neutrons, ie 7 and then it would be called carbon 14 (7+7=14) or 6 and then it would be called carbon 13 (7+6=13)

An isotope is an atom of element that has a different number of neutrons than the more common version. An example would be: the more common version of Uranium is Uranium-238, while another version is Uranium-235.