noun An organizer that papers are kept in, usually with an index tab, to be stored as a single unit in a filing cabinet.
I keep all my schoolwork in a yellow folder.
noun A virtual container in a computer's file system, in which files and other folders may be stored. The files and subfolders in a folder are usually related.
My essays are in the folder named "Schoolwork".
noun A machine or person that folds things.
noun A folding knife, typically a folding pocketknife.