The smelly green ghost in "Ghostbusters" is Slimer. He is a gluttonous, translucent green ghost first encountered by the Ghostbusters at New York's Sedgewick Hotel.
In the 1984 film, the character is not actually called Slimer by name. The crew referred to him as the "Onionhead Ghost," partly because an early version of the character was associated with an unpleasant smell. Ray Stantz describes him as an "ugly little spud" before Slimer covers Peter Venkman in ectoplasmic slime.
The name Slimer became established through the 1986 animated series "The Real Ghostbusters," where he became the team's mischievous mascot and occasional ally. The character's popularity also led to appearances in "Ghostbusters II," video games, merchandise, and the Hi-C Ecto Cooler drink. A common mix-up is confusing Slimer with Muncher, a different blue-gray ghost introduced in "Ghostbusters: Afterlife."