Definitions
1.PhraseThe normal course of an activity, particularly in circumstances that are out of the ordinary.
2.Phrase(The addition of quotations indicative of this usage is being sought:) The normal execution of standard functional operations within an organisation, particularly in contrast to a project or program which would introduce change (although that change may itself become business as usual).
3.Adjective(Should we delete this sense?) That is usual, expected, not extraordinary.