People often confuse this word with the term isotope. An isotope compares different versions of one single element. For example, carbon-12 and carbon-14 are isotopes of carbon.
The word nuclide is a more general label for any specific nucleus in the universe. You can compare a carbon nuclide directly to an oxygen nuclide. Every nuclide has a specific mass number.
The mass number is the total count of protons and neutrons. A specific nuclide is usually written using the element symbol and the mass number. Tracking these exact atoms is extremely important in nuclear chemistry.
Some nuclides are completely stable and last forever. Other nuclides are radioactive and slowly break apart over time. Nuclear physicists study them to track complex radioactive decay paths.
They also use the exact proton and neutron counts to calculate nuclear binding energy. This huge energy is what holds the atomic nucleus together so tightly. Knowing the exact nuclide tells you exactly how heavy the atom is. It also tells you exactly how the atom will behave in a nuclear reactor.