This is the name of the directory for the temporary files which are not accessible via HTTP. It chiefly contains the log files, database exports and most of the cache files.
Application: if you want to store the files in question elsewhere than in tmp/, you only need to change the value of this constant to be the new directory that you prefer (e.g. "tmp123456/").
This constant can only be modified directly in ecrire/inc_version.php.