A nuclide is an individual type of atom characterised by a specific atomic number (Z, the number of protons) and a specific mass number (A, the number of protons plus neutrons). The term is used when the exact nuclear composition matters, as in nuclear chemistry and the study of isotopes. Two atoms with the same Z but different A are different nuclides and are isotopes of the same element.

核素是由特定的原子序数(Z,即质子数)和特定的质量数(A,即质子加中子数)所表征的一类单个原子。当核的精确组成至关重要时(如在核化学和同位素研究中)便使用这一术语。Z 相同而 A 不同的两个原子是不同的核素,也是同种元素的同位素。