An abbreviation or abbrev is a string of characters that may be expanded to a longer string. The user can insert the abbrev string and find it replaced automatically with the expansion of the abbrev. This saves typing.
The set of abbrevs currently in effect is recorded in an abbrev table. Each buffer has a local abbrev table, but normally all buffers in the same major mode share one abbrev table. There is also a global abbrev table. Normally both are used.
An abbrev table is represented as an obarray. See Creating and Interning Symbols, for information about obarrays. Each abbreviation is represented by a symbol in the obarray. The symbol’s name is the abbreviation; its value is the expansion; its function definition is the hook function for performing the expansion (see Defining Abbrevs); and its property list cell contains various additional properties, including the use count and the number of times the abbreviation has been expanded (see Abbrev Properties).
Certain abbrevs, called system abbrevs, are defined by a major
mode instead of the user. A system abbrev is identified by its
non-nil :system property (see Abbrev Properties).
When abbrevs are saved to an abbrev file, system abbrevs are omitted.
See Saving Abbrevs in Files.
Because the symbols used for abbrevs are not interned in the usual obarray, they will never appear as the result of reading a Lisp expression; in fact, normally they are never used except by the code that handles abbrevs. Therefore, it is safe to use them in a nonstandard way.
If the minor mode Abbrev mode is enabled, the buffer-local variable
abbrev-mode is non-nil, and abbrevs are automatically
expanded in the buffer. For the user-level commands for abbrevs, see
Abbrev Mode in The GNU Emacs Manual.