GNU Emacs Lisp Reference Manual
Chapter 17: Minibuffers
A minibuffer is a special buffer that Emacs commands use to read
arguments more complicated than the single numeric prefix argument.
These arguments include file names, buffer names, and command names (as
in M-x). The minibuffer is displayed on the bottom line of the
screen, in the same place as the echo area, but only while it is in
use for reading an argument.