Ghouls
Ghouls are demons, a race of fallen Maiar, who are incorporeal and whose spirits take possess of bodies of dead Hildor (or more seldom Elves or even Orcs), the Term Wight is often used synonymous with Ghoul, but is sometimes also applied to normal animated Corpses or spirits of dead Hildor.
The term Ghoul (Har."Gul") is common in Harad or southern Middle-Earth.In the Westlands a specific type of Ghoul known as the Gaunt-men are better known.
Use in MERP
In MERP the term "Ghoul" is often used to categorize creatures that have degenerated under the influence of evil, but have not yet died but stretched theit lifespan in an unnatural way.By this definition the degenerated Stoor Gollum is labelled a Hobbit/Ghoul in several MERP Rulebooks.By this definition also other creatures as the Ring-Wights would fall into the "Ghoul" Category.On the other Hand "Gul" is used in the MERP Books as a specific Haradaic word for a class of Undead.
Other Systems
In the Lotr SBG a specific type of Undead known as Dimloki of Haradwaith are mentioned.