eintr: Permanent Installation
3.5 Install Code Permanently
============================
When you install a function definition by evaluating it, it will stay
installed until you quit Emacs. The next time you start a new session
of Emacs, the function will not be installed unless you evaluate the
function definition again.
At some point, you may want to have code installed automatically
whenever you start a new session of Emacs. There are several ways of
doing this:
• If you have code that is just for yourself, you can put the code
for the function definition in your ‘.emacs’ initialization file.
When you start Emacs, your ‘.emacs’ file is automatically evaluated
and all the function definitions within it are installed. 
Your ‘.emacs’ File Emacs Initialization.
• Alternatively, you can put the function definitions that you want
installed in one or more files of their own and use the ‘load’
function to cause Emacs to evaluate and thereby install each of the
functions in the files. Loading Files Loading Files.
• Thirdly, if you have code that your whole site will use, it is
usual to put it in a file called ‘site-init.el’ that is loaded when
Emacs is built. This makes the code available to everyone who uses
your machine. (See the ‘INSTALL’ file that is part of the Emacs
distribution.)
Finally, if you have code that everyone who uses Emacs may want, you
can post it on a computer network or send a copy to the Free Software
Foundation. (When you do this, please license the code and its
documentation under a license that permits other people to run, copy,
study, modify, and redistribute the code and which protects you from
having your work taken from you.) If you send a copy of your code to
the Free Software Foundation, and properly protect yourself and others,
it may be included in the next release of Emacs. In large part, this is
how Emacs has grown over the past years, by donations.
Info Catalog
eintr: Interactive Options
eintr: Writing Defuns
eintr: let