A nuclear species (also called a nuclide) is a specific configuration of protons and neutrons inside of a nucleus. This term also refers to how much energy those nucleons (collective term for protons and neutrons) have. Nuclear scientists talk about different nuclei being different nuclear species when those nuclei differ in any way; number of protons, neutrons, or configuration of energy.
Both of the previous examples changed the mass number. Both of the two nuclear species below are two different isobars (same mass number, different number of protons and neutrons individually) with a mass number of 40. This pair would also be two different nuclear species:
Slightly abstractly, putting one of the nucleons into a different configuration (same number of protons, same number of neutrons, but in different energy levels) would mean these two different nulcear species are two different nuclear isomers)):