TNS ADMIN

TNS_ADMIN is an environment variable that points to the directory where the SQL*Net configuration files (like sqlnet.ora and tnsnames.ora) are located.

Windows:

set TNS_ADMIN=%ORACLE HOME%\network\admin

Unix/ Linux:

export TNS_ADMIN=$ORACLE HOME/network/admin

Also, see TWO TASK, ORACLE HOME, ORACLE SID and ORACLE BASE.