meaning of abbreviations h and m in Na channels
Those initials were proposed by Hodgkin and Huxley in their original paper modeling the action potential in nerve cells: https://en.wikipedia.org/wiki/Hodgkin%E2%80%93Huxley_model  

They are not names for subunits, domains or motifs in the sodium channel, as your question may imply. Instead,


  n, m, and h are dimensionless quantities between 0 and 1 that are associated with potassium channel activation, sodium channel activation, and sodium channel inactivation, respectively.


However, there is no indication of what those initials mean (if anything), apparently being just letters they chose to name the variables in their equations.