Let's say my name is Margaret. But maybe it's spelled Margarett. What about if I go by Marge, which is basically the first five letters. Or maybe I go by Maggie. Or maybe I go by Meg. Though my name could be Megan. Or maybe it's Greta or Marjorie.
I say all that to ask this question: What is the difference between a derivitive, a diminutive, a variant, a short form, a pet form, a nickname, and any other word I'm missing? I just realized as I'm starting work on a book I'm writing, that these are among those words that I now what they mean, but I don't know what they mean either.