Why scientists care
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Their chemistry is very similar, while their masses and nuclear stability can differ.
One element can therefore have several nuclear versions without changing its place in the periodic table.
A concrete example
Hydrogen-1 and deuterium are both hydrogen because each nucleus has one proton, but deuterium also has one neutron.
Try to notice
Science becomes memorable when a definition is connected to something measurable or observable.