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The minibuffer is a part of Emacs used to read in complex commands or arguments.

The minibuffer is where Emacs commands read complicated arguments, such as file names, buffer names, Emacs command names, or Lisp expressions. It's called the "minibuffer" because it's a special-purpose buffer with a small amount of screen space. It is still an Emacs buffer, a first-class object, but it is a specialized one.