Are the terms Ghost Ring and Peep Sight interchangeable, or do they mean diferent things?

The terms are not synonymous but both are forms of aperture rear sights. If you remove the screw-in aperture from the usual Williams or Lyman receiver sight the larger ring that remains is what most folks describe as the ghost ring. This ring is so thin it effectively "disappears" when you look though it to focus on the front sight, hence the moniker "ghost ring".

Thank You.