— Function: define-abbrev abbrev-table name expansion &optional hook &rest props

This function defines an abbrev named name, in abbrev-table, to expand to expansion and call hook, with properties props (see Abbrev Properties). The return value is name. The :system property in props is treated specially here: if it has the value force, then it will overwrite an existing definition even for a non-system abbrev of the same name.

name should be a string. The argument expansion is normally the desired expansion (a string), or nil to undefine the abbrev. If it is anything but a string or nil, then the abbreviation expands solely by running hook.

The argument hook is a function or nil. If hook is non-nil, then it is called with no arguments after the abbrev is replaced with expansion; point is located at the end of expansion when hook is called.

If hook is a non-nil symbol whose no-self-insert property is non-nil, hook can explicitly control whether to insert the self-inserting input character that triggered the expansion. If hook returns non-nil in this case, that inhibits insertion of the character. By contrast, if hook returns nil, expand-abbrev (or abbrev-insert) also returns nil, as if expansion had not really occurred.

Normally, define-abbrev sets the variable abbrevs-changed to t, if it actually changes the abbrev. This is so that some commands will offer to save the abbrevs. It does not do this for a system abbrev, since those aren't saved anyway.

— User Option: only-global-abbrevs

If this variable is non-nil, it means that the user plans to use global abbrevs only. This tells the commands that define mode-specific abbrevs to define global ones instead. This variable does not alter the behavior of the functions in this section; it is examined by their callers.