What does "lacks a prefix" mean?

[BobD]

My elisp:

(defun blabba ()
  (defvar n 0)
)

byte-compile-file yields this:

  In blabba:
  test.el:1:8:Warning: global/dynamic var `n' lacks a prefix

What does my elisp lack?
Other than asking here, where can I learn what "lacks a prefix" means?