DROP DATABASE drops a database. It removes the catalog entries for the database and deletes the directory containing the data. It can only be executed by the database owner. Also, it cannot be executed while you or anyone else are connected to the target database. (Connect to template1 or any other database to issue this command.)
DROP DATABASE cannot be undone. Use it with care!
DROP DATABASE cannot be executed inside a transaction block.
This command cannot be executed while connected to the target database. Thus, it might be more convenient to use the program dropdb instead, which is a wrapper around this command.