Abbrevs can also have global definitions that are active in all major modes. The same abbrev can have a global definition and various mode-specific definitions for different major modes. A mode-specific definition for the current major mode overrides a global definition. Abbrevs can be defined interactively during the editing session. Lists of abbrev definitions can also be saved in files and reloaded in later sessions.
Some users keep extensive lists of abbrevs that they load in every session.
This command discards all abbrev definitions currently in effect, leaving a blank slate. This reads the abbrev itself using the minibuffer, and then defines it as an abbrev for one or more words before point. Use a numeric argument to say how many words before point should be taken as the expansion. An argument of zero to C-x a g means to use the contents of the region as the expansion of the abbrev being defined. Mode-specific abbrevs are active only in a particular major mode.