It is one of two or more atoms having the same atomic number but different mass numbers.
Isotopes are different types of atoms of the same chemical element , each having a different number of neutrons. Correspondingly, isotopes differ in mass number but not in atomic number. The difference in the number of nucleons comes from a difference how many neutrons are in the atomic nucleus. The number of protons is the same because that is what characterizes a chemical element.
Here is a video that describes more about Isotopes. Enjoy!
To add on, here is a short 20s video about an example of isotopes.
