Simple Definition:
Isotopes are different forms of a specific element that have the same number of protons but a different number of neutrons [An element is a type of substance that is made up of only one type of atom]. Isotopes of an element have the same number of protons in their nucleus, which defines the element, but a different number of neutrons, which changes the atomic mass. For example, carbon-12 and carbon-14 are isotopes of carbon. Carbon-12 has 6 protons and 6 neutrons, while carbon-14 has 6 protons and 8 neutrons.
Very Simple Definition:
An isotope is a type of atom that has the same number of protons as other atoms of the same element, but a different number of neutrons. For example, carbon-14 is an isotope of carbon that has 6 protons and 8 neutrons, while carbon-12 is an isotope of carbon that has 6 protons and 6 neutrons.