Quiesced state

Occasionally you might want to put a database in a state that allows only DBA transactions, queries, fetches, or PL/SQL statements. Such a state is referred to as a quiesced state, in the sense that no ongoing non-DBA transactions, queries, fetches, or PL/SQL statements are running in the system.

ALTER SYSTEM QUIESCE RESTRICTED;

ALTER SYSTEM UNQUIESCE;

select active_state from v$instance;

NORMAL: Normal unquiesced state.
QUIESCING: Being quiesced, but some non-DBA sessions are still active.
QUIESCED: Quiesced; no non-DBA sessions are active or allowed.

http://download.oracle.com/docs/cd/B19306_01/server.102/b14231/start.htm#sthref603