Altimeter setting in millibars of mercury  Nobody has been able to tell me the origin of the term.  I did a Google search.  Q in Morse code means "station" so QNH means "station normal height." 

 

 

An obsolete term you  still see now and then is QFE which setting the altimeter to zero at the field elevation to know your elevation above the ground directly with no mental math.  Probably used during training on flat ground long, long ago.