— Function: define-abbrev table name expansion &optional hook count system-flag

This function defines an abbrev named name, in table, to expand to expansion and call hook. The return value is name.

The value of count, if specified, initializes the abbrev's usage-count. If count is not specified or nil, the use count is initialized to zero.

The argument 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 also returns nil, as if expansion had not really occurred.

If system-flag is non-nil, that marks the abbrev as a “system” abbrev with the system-type property.

Normally the function 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 won't be 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.