elisp: Autoloading
9.2.8 Autoloading
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The “autoload” feature allows you to call a function or macro whose
function definition has not yet been loaded into Emacs. It specifies
which file contains the definition. When an autoload object appears as
a symbol’s function definition, calling that symbol as a function
automatically loads the specified file; then it calls the real
definition loaded from that file. The way to arrange for an autoload
object to appear as a symbol’s function definition is described in 
Autoload.
Info Catalog
elisp: Special Forms
elisp: Forms