Atoms of the same element that have the same number of protons but a different number of neutrons.

Here's a breakdown:

* Same element: Isotopes belong to the same element, meaning they have the same atomic number (number of protons).

* Different number of neutrons: The defining characteristic of isotopes is that they have a different number of neutrons. This leads to a different atomic mass.

For example, Carbon-12 and Carbon-14 are both isotopes of carbon. They both have 6 protons, but Carbon-12 has 6 neutrons, while Carbon-14 has 8 neutrons.