GNU Emacs Lisp Reference Manual
A file of saved abbrev definitions is actually a file of Lisp code. The abbrevs are saved in the form of a Lisp program to define the same abbrev tables with the same contents. Therefore, you can load the file with load (see How Programs Do Loading). However, the function quietly-read-abbrev-file is provided as a more convenient interface.
User-level facilities such as save-some-buffers can save abbrevs in a file automatically, under the control of variables described here.
write-abbrev-file. If filename is nil, the file specified in abbrev-file-name is used. save-abbrevs is set to t so that changes will be saved. This function does not display any messages. It returns nil.
nil value for save-abbrev means that Emacs should save abbrevs when files are saved. abbrev-file-name specifies the file to save the abbrevs in.nil by defining or altering any abbrevs. This serves as a flag for various Emacs commands to offer to save your abbrevs.nil.