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connect [SQL*Plus]

connect username
connect username@instance-name

connect username/password
connect username/password@instance-name

connect / as sysdba
connect / as sysoper

connect proxy-username[username]
connect proxy-username[username]@instance-name

connect proxy-username[username]/password
connect proxy-username[username]/password@instance-name

connect / as sysdba
connect / as sysoper
Allows to connect to an Oracle instance. That is, it creates a new session.
New in Oracle 10g: Oracle executes also glogin.sql and login.sql on a connect. This is different from 9i and earlier versions where login.sql and glogin.sql were only executed upon starting SQL*Plus.

Proxy user

See here.