The MySQL daemon/engine on your server has run out of (allowed) space for storing its own database files. If the filename/path mentioned includes "/tmp/" in it, then the error is likely referring to temporary/swap files instead of main database files.SOLUTION:
If you are the server/host administrator and can control disk configurations/permissions on your server, either reconfigure MySQL to use another location with more space available; or add more space to the indicated volume/mount-point (in the example above, that would be /var); or relax security surrounding the /var/tmp folder for the MySQL user.
If you are not the server administrator, you need to report the error to your host/web-company so that they can resolve the situation for you. This is a good time to remind you to do regular backups... You should be sure to double-check all your data after encountering such an error, and having a good backup on hand could save you tons of trouble.