The DATABASE_HOST variable is a special case of the HOST variable that specifies the host name of the server where the database is running.
Hostname specification
None
The database host is specified by adding database to an existing HOST_localhost property, or adding a HOST_localhost property if it is not already there.
This site has the database running on the same box as all other services:
HOST_localhost=app cvs subversion database
This site has the database running on its own separate box:
HOST_localhost=app cvs subversion HOST_<database_host>=database