GNU Emacs Lisp Reference Manual
GNU Emacs Lisp Reference Manual
An overlay specifies temporary alteration of the display appearance of a part of a buffer. It contains markers delimiting a range of the buffer, plus a property list (a list whose elements are alternating property names and values). Overlays are used to present parts of the buffer temporarily in a different display style. They have no read syntax, and print in hash notation, giving the buffer name and range of positions.
See Overlays, for how to create and use overlays.