What does the z stand for in Dragon Ball Z

I'm not sure for the Z, but GT stands for "gomenasai toriyama-san" it means "I'm sorry mr. toriyama" because a student of his made that series against toriyama's permission.
dragon ball z stands for z fighters and dragon ball gt stands for grand tour because it is the last series. dragon ball af stands for April fools some people think it stands for after future but it dosent some other people belive it stands for after freiza but it is make all make belive and fan made romors
The Z was placed into the name because it was the last letter of the alphabet and it was the end of the Dragonball series, i know this sounds retarded but thats what Akiri Toryama said in an interview to shonen jump in 2003