Mirror nuclei is a term that refers to two different elemental isotopes with atomic numbers having difference of one, but with the same combined number of protons and neutrons that is with same mass number .

If the sum of the number of proton

Proton

The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....

s (Z) plus the number of neutrons (N) is the same, then the two nuclide

Nuclide

A nuclide is an atomic species characterized by the specific constitution of its nucleus, i.e., by its number of protons Z, its number of neutrons N, and its nuclear energy state....

s are sometimes described as "mirror isotopes." However, they are entirely different elements, with different chemical properties, and cannot be properly be described as "isotopes" (atoms of the same chemical element with slightly-different masses, due to a differing number of neutrons).

Examples of mirror nuclei:
H with mass number 3 and He with mass number 3 . here both the species have same mass number but atomic number difference of 1 .