off | else | syntax enable | endif

To put this into a mapping, you can use: >
   :map <F7> :if exists("g:syntax_on") <Bar>
	\   syntax off <Bar>
	\ else <Bar>
	\   syntax enable <Bar>
	\ endif <CR>
[using the |<>| notation, type this literally]

Details:
The ":syntax" commands are implemented by sourcing a file.  To see exactly how
this works, look in the file:
    command		file ~
    :syntax enable	$VIMRUNTIME/syntax/syntax.vim
    :syntax on		$VIMRUNTIME/syntax/syntax.vim
    :syntax manual	$VIMRUNTIME/syntax/manual.vim
    :syntax off		$VIMRUNTIME/syntax/nosyntax.vim
Also see |syntax-loading|.

NOTE: If displaying long lines is slow and switching off syntax highlighting
makes it fast, consider setting the 'synmaxcol' option to a lower value.

==============================================================================
2. Syntax files						*:syn-files*

The syntax and highlighting commands for one language are normally stored in
a syntax file.	The name convention is: "{name}.vim".  Where {name} is the
name of the language, or an abbreviation (to fit the name in 8.3 characters,
a requirement in case the file is used on a DOS filesystem).
Examples:
	c.vim		perl.vim	java.vim	html.vim
	cpp.vim		sh.vim		csh.vim

The syntax file can contain any Ex commands, just like a vimrc file.  But
the idea is that only commands for a specific language are included.  When a
language is a superset of another language, it may include the other one,
for example, the cpp.vim file could include the c.vim file: >
   :so $VIMRUNTIME/syntax/c.vim

The .vim files are normally loaded with an autocommand.  For example: >
   :au Syntax c	    runtime! syntax/c.vim
   :au Syntax cpp   runtime! syntax/cpp.vim
These commands are normally in the file $VIMRUNTIME/syntax/synload.vim.


MAKING YOUR OWN SYNTAX FILES				*mysyntaxfile*

When you create your own syntax files, and you want to have Vim use these
automatically with ":syntax enable", do this:

1. Create your user runtime directory.	You would normally use the first item
   of the 'runtimepath' option.  Example for Unix: >
	mkdir ~/.config/nvim

2. Create a directory in there called "syntax".  For Unix: >
	mkdir ~/.config/nvim/syntax

3. Write the Vim syntax file.  Or download one from the internet.  Then write
   it in your syntax directory.  For example, for the "mine" syntax: >
	:w ~/.config/nvim/syntax/mine.vim

Now you can start using your syntax file manually: >
	:set syntax=mine
You don't have to exit Vim to use this.

If you also want Vim to detect the type of file, see |new-filetype|.

If you are setting up a system with many users and you don't want each user
to add the same syntax file, you can use another directory from 'runtimepath'.


ADDING TO AN EXISTING SYNTAX FILE		*mysyntaxfile-add*

If you are mostly satisfied with an existing syntax file, but would like to
add a few items or change the highlighting, follow these steps:

1. Create your user directory from 'runtimepath', see above.

2. Create a directory in there called "after/syntax".  For Unix: >
	mkdir -p ~/.config/nvim/after/syntax

3. Write a Vim script that contains the commands you want to use.  For
   example, to change the colors for the C syntax: >
	highlight cComment ctermfg=Green guifg=Green

4. Write that file in the "after/syntax" directory.  Use the name of the
   syntax, with ".vim" added.  For our C syntax: >
	:w ~/.config/nvim/after/syntax/c.vim

That's it.  The next time you edit a C file the Comment color will be
different.  You don't even have to restart Vim.

If you have multiple files, you can use the filetype as the directory name.
All the "*.vim" files in this directory will be used, for example:
	~/.config/nvim/after/syntax/c/one.vim
	~/.config/nvim/after/syntax/c/two.vim


REPLACING AN EXISTING SYNTAX FILE			*mysyntaxfile-replace*

If you don't like a distributed syntax file, or you have downloaded a new
version, follow the same steps as for |mysyntaxfile| above.  Just make sure
that you write the syntax file