CONNECT username/password@tns_entryThe italicized portion is a connect string. It contains the username, the password and the TNSNAMES.ORA entry to point to a remote database.
The term "host string" is not defined in the Oracle documentation of network terms. The closest in the glossary is "host naming," which is a method of defining database servers in your network.
The only other thing I could think of that "host string" could mean in Oracle terminology is that the TNSNAMES.ORA and LISTENER.ORA configuration files can contain an entry defining the host of the database.
Most Oracle people I know do not use the term "host string," so I am unfamiliar with how you are using it. If you have further information and need additional clarification, please let me know.
This was first published in May 2005