What is the definition of Isotopes?

Isotopes are different forms of an atom - they have the same number of electrons and protons but a different number of neutrons.

Isotopes of the same element will have the same chemical properties as this is determined by the number of electrons.

Isotopes of the same elements will have different mass numbers (protons and neutrons) but the same atomic number (protons or electrons).

An AQA definition is Isotopes are different forms of an atom - they have the same number of electrons and protons but a different number of neutrons. So Carbon 12 and Carbon 14 are isotopes.