What does "idle instance" mean when executing "sqlplus / as sysdba" command?

My Oracle SID environment is:

ORACLE_SID="ORCL5" When I didn't start ORCL5 instance, executing "sqlplus system/system" generated errors:

$ sqlplus system/system

SQL*Plus: Release 12.1.0.2.0 Production on Tue Feb 23 03:04:34 2016

Copyright (c) 1982, 2014, Oracle. All rights reserved.

ERROR:
ORA-01034: ORACLE not available
ORA-27101: shared memory realm does not exist Linux-x86_64 Error: 2: No such file or directory Additional information: 3640
Additional information: 1813205355
Process ID: 0
Session ID: 0 Serial number: 0

While executing "sqlplus / as sysdba" prompts "Connected to an idle instance.":

$ sqlplus / as sysdba

SQL*Plus: Release 12.1.0.2.0 Production on Tue Feb 23 03:04:55 2016

Copyright (c) 1982, 2014, Oracle. All rights reserved.

Connected to an idle instance.

SQL> I am a little confused of What the "idle instance" is here.

Could anyone help to explain it? Thanks very much in advance!

Best Regards
Nan Xiao

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