Summary: Pericoronitis also known as operculitis, is inflammation of the soft tissues surrounding the crown of a partially erupted tooth, including the gingiva (gums) and the dental follicle. The soft tissue covering a partially erupted tooth is known as an operculum, an area which can be difficult to access with normal oral hygiene methods. The synonym operculitis technically refers to inflammation of the operculum alone.
Reference Material:
https://en.wikipedia.org/wiki/Pericoronitis