Whenever Billy says the word “Shazam!”, a magical lightning bolt strikes him, and he becomes the adult Captain Marvel, though he still has Billy’s intellect and personality.
Unlike the film’s lead antagonist (more on him later), no actor has been announced yet to play Billy or Shazam.
To transform back into Billy Batson, Captain Marvel just has to say the same word.
As a result, the various series over the years starring him have had titles circling around the word “Shazam!”