mysqldump all-databases option

--all-databases is a mysqldump option that is used when you want to include all the databases in your dump.

mysqldump --all-databases > my_backup.sql

When executing the command from above, the dump file will contain all the databases. Before each database, a CREATE STATEMENT is written. In case you want to also write a DROP DATABASE statement before each CREATE DATABASE, use the --add-drop-database option.

# Write a DROP DATABASE statement before each CREATE DATABASE statement
mysqldump --all-databases --add-drop-database > my_backup.sql

If you want to find more details about how to backup one, more or all your databases at once, this article covers that.