# What are isodiaphers?

We know that isodiaphers have same difference in neutrons and protons, but I encountered another definition:

Atoms which have same isotopic axis are called isodiaphers.

I don't know what it means by "isotopic axis". Please try to explain in simple words.

Isotopes are nuclides with a common number of protons $Z$.
Isobars are nuclides with a common number of nucleons $A$.
Isotones are nuclides with a common number of neutrons $N$.
Isodiaphers are nuclides with a common neutron excess $N-Z=A-2Z$.