An isotope is when an alement has a different number of neutrons in the nucleus, but the same number of protons. It comes from the Greek root “iso” meaning equal, and “topos” meaning place.
As an example there is Carbon 12, 13 and 14. Given carbon has six protons, this means each isotope has 6, 7 and 8 neutrons respectively. The measurement of the “Atomic Mas Unit” (AMU) is 1/12 of the mass of an atom of C12.