major-mode

major-mode is a variable defined in `C source code'.
Its value is

fundamental-mode

global value is the same.

• Automatically becomes buffer-local when set.

Documentation:
Symbol for current buffer's major mode.
The default value (normally `fundamental-mode') affects new buffers.
A value of nil means to use the current buffer's major mode, provided
it is not marked as "special".

When a mode is used by default, `find-file' switches to it before it
reads the contents into the buffer and before it finishes setting up
the buffer. Thus, the mode and its hooks should not expect certain
variables such as `buffer-read-only' and `buffer-file-coding-system'
to be set up.

You can customize this variable.