Here is my lay definition of an isotope:

"An atom is made up of the nucleus and the electrons that surround it. In the nucleus you have protons and neutrons. If you change the number of protons, you change what element it is, so you can change Oxygen to Carbon or Nitrogen if you add some protons. If you change the number of neutrons, you don't change the element, you change the isotope. The isotope is kinda what type of the element the atom is.

The reason we talk about isotopes at all is because for nuclear reactions you need specific elements like uranium, but on top of that you need specific isotopes of uranium because only some isotopes act the right way for nuclear reactions to take place."