— User Option: abbrev-mode

If this variable is non-nil, abbrevs are automatically expanded in the buffer. If the value is nil, abbrevs may be defined, but they are not expanded automatically.

This variable automatically becomes buffer-local when set in any fashion.

— Variable: default-abbrev-mode

This is the value of abbrev-mode for buffers that do not override it. It is the same as (default-value 'abbrev-mode).