How is the distance between m/z axis values in mass spectrums called?
I wanted to back up my comment and make sure it's justified, so I decided to flip through Gross'Mass spectrometry: a textbook [1] and it looks like there are indeed basically two possible variations, including distance between $m/z$ values, as you suggested:


{Distance | difference | mass difference} between {peaks | signals | peak tips}
{Difference in $m/z$ | $m/z$ difference | $Δ(m/z)$}


Both refer to describing isotopic distribution as well as to reading and interpreting the spectra.

Selected parts touching standardization (emphasis mine):

From [1, p. 11]:


  The distance between peaks on that axis has the meaning of a neutral loss from
  the ion at higher $m/z$ to produce the fragment ion at lower $m/z.$ Therefore, the
  amount of this neutral loss is given as “$x~\pu{u}$”, where the symbol $\pu{u}$ stands for unified atomic mass. It is important to notice that the mass of the neutral is only reflected by the difference between the corresponding $m/z$ values, i.e., $Δ(m/z).$ This is because the mass spectrometer detects only charged species, i.e., the charge-retaining group of a fragmenting ion.


Further summary from [1, p. 20]:


  1.7.1 Basic Terminology in Describing Mass Spectra
  
  […]
  
  
  Ranges in spectra or ranges set in operating a mass spectrometer are to be
  referred to in the form of “$m/z~10–100$” or “$m/z~10$ to $m/z~100$”.
  
  
  […]
  
  
  Neutral losses (rarely called dark matter […]) are exclusively recognized from
  the distance between peaks expressed in terms of the difference $Δ(m/z).$ The
  mass of the corresponding neutral is then given in units of $\mathrm{u}.$
  


References


Gross, J. H. Mass Spectrometry: A Textbook, 3rd ed.; Springer International Publishing: Cham, Switzerland, 2017. ISBN 978-3-319-54397-0.