Lots of words are used in multiple ways ("overloaded" if you like) and this is one of them.
I've concluded that it's pointless to try to establish the "correct" definition for such words-- we are not the Academie Francaise, after all, or the OED; we can, however, decide how we will use the term in a particular document, report or whatever (and make that clear.) Or, rather than use really horribly overloaded terms, we can avoid them and pick another less popular term, and provide a nice clear definition in a glossary appendix.
With that preamble, I think a useful distinction I've heard in the discussion (e-mail and oral) in the SOA-RM TC is that "enterprise" should be used (in SOA-RM TC documents) to denote organizations (including joint ventures, associations, programs, temporary organizations) with a single purpose (and not merely "large" organizations, for example, despite my own first thought on the topic.)
If we adopt that ("common purpose") idea, then we should definitely state the definition in any document that uses it.