Isotopes, Isobars, Isotones, and Isomers

Isotopes
Atoms having the same number of protons (Z) but different number of neutrons (N) are isotopes.

Isobars
Atoms having the same mass number (A) are isobars.

Isotones
Atoms having the same number of neutrons (N) but a different number of protons (Z) are isotones.

Isomers
Atoms of the same element (same Z and N) but which are in different excited states are isomers.